diff --git a/VERSION b/VERSION
index 57dc14fa..0c55b9cf 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-0.74.0
\ No newline at end of file
+0.75.0
\ No newline at end of file
diff --git a/backend/alembic/versions/0074_download_credentials.py b/backend/alembic/versions/0074_download_credentials.py
new file mode 100644
index 00000000..b6d7d70c
--- /dev/null
+++ b/backend/alembic/versions/0074_download_credentials.py
@@ -0,0 +1,46 @@
+"""download_credentials (S6: per-user, per-host yt-dlp cookie/auth store)
+
+Each user stores their OWN login cookies (Netscape cookies.txt) for an auth-walled download host
+(Facebook, Instagram, …), Fernet-encrypted at rest. A download of a yt-dlp-supported, auth-walled
+link on that host is retried with the acting user's cookie. Unique per (user_id, host_key) so a
+user holds at most one credential per host. Empty for every existing install; forward-only in
+practice (a downgrade just drops the table + its secrets).
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0074_download_credentials"
+down_revision: Union[str, None] = "0073_saved_view_source"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "download_credentials",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column(
+            "user_id",
+            sa.Integer(),
+            sa.ForeignKey("users.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column("host_key", sa.String(length=64), nullable=False),
+        sa.Column("cookies_enc", sa.Text(), nullable=False),
+        sa.Column(
+            "created_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False
+        ),
+        sa.Column(
+            "updated_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False
+        ),
+        sa.UniqueConstraint("user_id", "host_key", name="uq_download_credential"),
+    )
+    op.create_index("ix_download_credentials_user_id", "download_credentials", ["user_id"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_download_credentials_user_id", table_name="download_credentials")
+    op.drop_table("download_credentials")
diff --git a/backend/alembic/versions/0075_playlist_sync_baseline.py b/backend/alembic/versions/0075_playlist_sync_baseline.py
new file mode 100644
index 00000000..b786fd60
--- /dev/null
+++ b/backend/alembic/versions/0075_playlist_sync_baseline.py
@@ -0,0 +1,26 @@
+"""playlist synced_video_ids (S7: 3-way merge baseline for two-way YouTube sync)
+
+The one-directional "local wins" push is replaced by a 3-way merge that classifies each item
+against the last-synced state. That needs the raw ordered id set as a baseline, not just the
+`synced_fingerprint` hash. Add a nullable `synced_video_ids` (JSON list) column; NULL on every
+existing row means "no baseline yet" — the first merge then unions losslessly and records one.
+Forward-only in practice (a downgrade just drops the column).
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0075_playlist_sync_baseline"
+down_revision: Union[str, None] = "0074_download_credentials"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("playlists", sa.Column("synced_video_ids", sa.Text(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("playlists", "synced_video_ids")
diff --git a/backend/app/audit.py b/backend/app/audit.py
index cba72f66..1578a656 100644
--- a/backend/app/audit.py
+++ b/backend/app/audit.py
@@ -44,6 +44,9 @@ class AuditAction:
     SYNC_RESUME = "sync_resume"
     # Scheduler job outcome / run-summary (actor_id set = a manual "run now"; NULL = automatic)
     JOB_RUN = "job_run"
+    # Per-user download cookie/auth credential set or removed (the value is NEVER recorded)
+    DOWNLOAD_CREDENTIAL_SET = "download_credential_set"
+    DOWNLOAD_CREDENTIAL_DELETE = "download_credential_delete"
     # The log itself was cleared (a single row survives the clear as tamper-evidence)
     AUDIT_CLEAR = "audit_clear"
 
diff --git a/backend/app/config.py b/backend/app/config.py
index edb511eb..d48d6de9 100644
--- a/backend/app/config.py
+++ b/backend/app/config.py
@@ -186,6 +186,21 @@ class Settings(BaseSettings):
     download_supervisor_poll_seconds: int = 3
     # Grace between SIGTERM and SIGKILL when stopping a child (matches run_ffmpeg's escalation).
     download_kill_grace_seconds: int = 5
+    # --- worker-child resource de-prioritization ---
+    # The download/edit child (yt-dlp + ffmpeg) shares ONE LAN box with the API, Postgres, and Plex
+    # playback. Without a priority handicap a CPU re-encode pegs every core for ~30s and starves web
+    # requests + playback until it finishes. The child lowers its OWN priority at startup — inherited
+    # by every ffmpeg/yt-dlp descendant — so any foreground work always wins. Each knob is best-effort
+    # and independently disable-able with 0.
+    download_child_nice: int = 10  # CPU nice increment applied to the child; 0 = don't renice
+    download_child_ionice_class: int = 3  # best-effort IO class (3 = idle); 0 = leave IO untouched
+    # Cap the browser-safe re-encode's encoder threads so a transcode can't claim every core. 0 =
+    # reserve one core for foreground work (max(1, cpu_count - 1)); >0 pins exactly that many.
+    download_ffmpeg_threads: int = 0
+    # Throttle the retention GC's whole-tree orphan file sweep so its rglob can't monopolize disk IO
+    # in the API process: sleep `pause_ms` after every `batch` files touched. pause_ms=0 disables.
+    download_gc_sweep_pause_ms: int = 20
+    download_gc_sweep_batch: int = 400
     # Total cache size cap across ALL users (bytes). When >0 and exceeded, the GC evicts the
     # least-recently-accessed ready assets until under it. 0 = unlimited (disk is the limit).
     download_total_max_bytes: int = 0
diff --git a/backend/app/downloads/credentials.py b/backend/app/downloads/credentials.py
new file mode 100644
index 00000000..af5b79fe
--- /dev/null
+++ b/backend/app/downloads/credentials.py
@@ -0,0 +1,258 @@
+"""Per-user, per-host download cookies — the single source of truth for "does the acting user's
+cookie apply to this download, and how does that keep the result private".
+
+Two call sites depend on ONE consistent answer (house rule: an invariant lives in shared code, not
+in prose):
+  * `enqueue` salts the MediaAsset cache signature with the user id WHEN a cookie applies, so the
+    authed download becomes a PRIVATE asset that is never cache-served to another user.
+  * the worker attaches the cookie file to yt-dlp ONLY when a cookie still applies AND the asset it
+    is downloading into is that private, salted identity — so a cookie can never poison a shared
+    asset (the load-bearing cross-user privacy fix).
+
+Both go through `cookie_salt` / `authed_format_sig` / `host_key_for_url` here, so they can't drift.
+
+Security controls enforced here:
+  * **host scope (SEC-12)** — `host_key_for_url` matches the download URL's netloc against a curated
+    registry via the netloc-scoped `_host_is`, so a cookie is only ever offered to the exact site it
+    belongs to; an arbitrary `?url=` (or a look-alike host) resolves to no host_key and gets no
+    cookie.
+  * **secret handling** — cookies are Fernet-encrypted at rest; the blob is only ever decrypted
+    in-worker into a 0600 file that is unlinked in a `finally`, and is never returned by a route,
+    logged, or written to a job's error/snapshot.
+"""
+
+import logging
+import os
+from pathlib import Path
+
+from sqlalchemy import select
+from sqlalchemy.orm import Session
+
+from app.downloads import formats
+from app.downloads.adapters import _host_is
+from app.models import DownloadCredential
+from app.security import decrypt_optional, encrypt
+
+log = logging.getLogger("siftlode.downloads.credentials")
+
+
+class HostSpec:
+    """A supported cookie host: a short canonical `key` (stored on the credential + folded into the
+    cache salt), a human `label` for the UI, and the netloc `domains` it matches."""
+
+    __slots__ = ("key", "label", "domains")
+
+    def __init__(self, key: str, label: str, domains: tuple[str, ...]):
+        self.key = key
+        self.label = label
+        self.domains = domains
+
+
+# Curated registry (user's call: seed the common auth-walled hosts now). One `key` can cover several
+# domains of the same site. Add a host by adding a row — the FE dropdown, host-scope guard, and salt
+# all derive from this, so there is nothing else to wire up.
+SUPPORTED_HOSTS: tuple[HostSpec, ...] = (
+    HostSpec("facebook", "Facebook", ("facebook.com", "fb.watch")),
+    HostSpec("instagram", "Instagram", ("instagram.com",)),
+    HostSpec("twitter", "X (Twitter)", ("twitter.com", "x.com")),
+    HostSpec("tiktok", "TikTok", ("tiktok.com",)),
+    HostSpec("reddit", "Reddit", ("reddit.com",)),
+)
+
+_BY_KEY: dict[str, HostSpec] = {h.key: h for h in SUPPORTED_HOSTS}
+
+
+def is_supported_host(host_key: str) -> bool:
+    return host_key in _BY_KEY
+
+
+def host_label(host_key: str) -> str:
+    spec = _BY_KEY.get(host_key)
+    return spec.label if spec else host_key
+
+
+def host_key_for_url(url: str | None) -> str | None:
+    """The supported host_key whose domains own `url`'s netloc, or None. Netloc-scoped (via
+    `_host_is`), so a look-alike host ('myfacebook.com', 'facebook.com.evil') never matches. When
+    several specs could match, the most-specific (longest matched domain) wins so the result is
+    deterministic across the enqueue and worker call sites."""
+    if not url:
+        return None
+    from urllib.parse import urlparse
+
+    host = urlparse(url).netloc.lower()
+    if not host:
+        return None
+    best_key: str | None = None
+    best_len = -1
+    for spec in SUPPORTED_HOSTS:
+        for dom in spec.domains:
+            if _host_is(host, dom) and len(dom) > best_len:
+                best_key, best_len = spec.key, len(dom)
+    return best_key
+
+
+def cookie_salt(user_id: int, host_key: str) -> str:
+    """The cache-signature salt for a cookie-authed download. Keyed by user id + host so the authed
+    asset is private to that user (never dedup-shared) and stable across that user's own re-adds."""
+    return f"cookie:{user_id}:{host_key}"
+
+
+def authed_format_sig(spec: dict, user_id: int, host_key: str) -> str:
+    """The salted `format_sig` an authed download for (user, host) resolves to — the private asset
+    identity. Both enqueue (to create it) and the worker (to recognise it) compute it HERE."""
+    return formats.format_sig(spec, salt=cookie_salt(user_id, host_key))
+
+
+def salt_for_enqueue(db: Session, user_id: int, url: str | None) -> str | None:
+    """At enqueue: the cache salt to apply if this user has a cookie for `url`'s host, else None (a
+    normal shared download). Returns the salt string so the caller passes it straight to
+    `format_sig(spec, salt=...)`."""
+    host_key = host_key_for_url(url)
+    if host_key is None:
+        return None
+    if get_credential(db, user_id, host_key) is None:
+        return None
+    return cookie_salt(user_id, host_key)
+
+
+# --- storage (CRUD) ---------------------------------------------------------------------------
+
+
+def get_credential(db: Session, user_id: int, host_key: str) -> DownloadCredential | None:
+    return db.execute(
+        select(DownloadCredential).where(
+            DownloadCredential.user_id == user_id, DownloadCredential.host_key == host_key
+        )
+    ).scalar_one_or_none()
+
+
+class InvalidCookies(ValueError):
+    """The pasted cookies aren't a usable Netscape cookies.txt — surfaced as a 400 by the route."""
+
+
+def validate_cookies(raw: str) -> str:
+    """Sanity-check a pasted cookies.txt (Netscape format) and return the cleaned text. yt-dlp reads
+    the Netscape format: comment/`#` lines plus TAB-separated cookie lines (domain, flag, path,
+    secure, expiry, name, value). We don't fully parse — just reject obvious non-cookie paste (empty,
+    or no tab-separated data line) so a mistaken paste fails HERE with a clear message rather than
+    silently producing a cookie file yt-dlp ignores."""
+    text = (raw or "").strip()
+    if not text:
+        raise InvalidCookies("empty")
+    has_data_line = any(
+        line.strip() and not line.lstrip().startswith("#") and "\t" in line
+        for line in text.splitlines()
+    )
+    if not has_data_line:
+        raise InvalidCookies("format")
+    # Netscape cookie files must end with a newline for some parsers; normalise.
+    return text if text.endswith("\n") else text + "\n"
+
+
+def upsert_credential(db: Session, user_id: int, host_key: str, raw: str) -> DownloadCredential:
+    """Create or replace this user's cookie blob for `host_key`. Caller has validated host_key is
+    supported and `raw` via `validate_cookies`. Does NOT commit (the route commits with the audit
+    row so the trail can't drift). The blob is encrypted here; the plaintext never leaves this call."""
+    cred = get_credential(db, user_id, host_key)
+    enc = encrypt(raw)
+    if cred is None:
+        cred = DownloadCredential(user_id=user_id, host_key=host_key, cookies_enc=enc)
+        db.add(cred)
+    else:
+        cred.cookies_enc = enc
+    return cred
+
+
+def delete_credential(db: Session, user_id: int, host_key: str) -> bool:
+    """Delete this user's cookie for `host_key`. Returns whether a row existed. Does NOT commit."""
+    cred = get_credential(db, user_id, host_key)
+    if cred is None:
+        return False
+    db.delete(cred)
+    return True
+
+
+def status_list(db: Session, user_id: int) -> list[dict]:
+    """The supported hosts with this user's configured/updated state — the write-only Settings view.
+    Never includes the blob."""
+    have = {
+        c.host_key: c
+        for c in db.execute(
+            select(DownloadCredential).where(DownloadCredential.user_id == user_id)
+        ).scalars()
+    }
+    out = []
+    for spec in SUPPORTED_HOSTS:
+        cred = have.get(spec.key)
+        out.append(
+            {
+                "host_key": spec.key,
+                "label": spec.label,
+                "configured": cred is not None,
+                "updated_at": cred.updated_at.isoformat() if cred is not None else None,
+            }
+        )
+    return out
+
+
+# --- worker: resolve + write the cookie file --------------------------------------------------
+
+
+def resolve_cookiefile(
+    db: Session,
+    user_id: int,
+    url: str | None,
+    asset_format_sig: str,
+    spec: dict,
+    staging: Path,
+) -> Path | None:
+    """In the worker child: the cookies.txt to hand yt-dlp for this download, or None.
+
+    The double gate that prevents cross-user leakage:
+      1. a credential must still exist for `url`'s host (host-scoped via `host_key_for_url`), AND
+      2. the asset being downloaded into must BE this user's private, salted identity
+         (`asset_format_sig == authed_format_sig(...)`).
+
+    If (2) fails the asset is the SHARED (unsalted) one — attaching the cookie would poison the
+    global cache with a private-group video, so we attach nothing. An unreadable blob (rotated key →
+    InvalidToken) also yields None; the download then fails honestly on the login wall (S3's
+    `login_required`)."""
+    host_key = host_key_for_url(url)
+    if host_key is None:
+        return None
+    cred = get_credential(db, user_id, host_key)
+    if cred is None:
+        return None
+    if asset_format_sig != authed_format_sig(spec, user_id, host_key):
+        return None  # shared asset — never attach a cookie to a non-private identity
+    raw = decrypt_optional(cred.cookies_enc)
+    if not raw:
+        log.warning(
+            "download cookie for user %s host %s could not be decrypted (rotated key?) — "
+            "proceeding without it",
+            user_id,
+            host_key,
+        )
+        return None
+    return _write_cookiefile(raw, staging)
+
+
+def _write_cookiefile(raw: str, staging: Path) -> Path:
+    """Write the cookie blob to a private 0600 file under `staging/.auth/` and return its path.
+
+    Deliberately NOT in the staging root: the download retry loop unlinks every file it finds there,
+    and `_produced_file` picks the largest file there as the output — either would corrupt a
+    cookies.txt in the root. A `.auth` SUBDIR survives both (they skip directories) and is still
+    removed by the parent worker's staging cleanup on a hard kill. The caller unlinks it in a
+    `finally` on the normal/exception paths."""
+    auth_dir = staging / ".auth"
+    auth_dir.mkdir(mode=0o700, exist_ok=True)
+    path = auth_dir / "cookies.txt"
+    # Open with 0600 from the start (don't create world-readable then chmod).
+    fd = os.open(path, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600)
+    try:
+        os.write(fd, raw.encode("utf-8"))
+    finally:
+        os.close(fd)
+    return path
diff --git a/backend/app/downloads/edit.py b/backend/app/downloads/edit.py
index 7f4d6a7a..a4a37b8d 100644
--- a/backend/app/downloads/edit.py
+++ b/backend/app/downloads/edit.py
@@ -25,6 +25,7 @@ import hashlib
 import json
 import logging
 import math
+import os
 import queue
 import shutil
 import subprocess
@@ -32,9 +33,21 @@ import threading
 import time
 from pathlib import Path
 
+from app.config import settings
+
 log = logging.getLogger("siftlode.edit")
 
 
+def encode_thread_cap() -> int:
+    """How many encoder threads a browser-safe re-encode may use. `download_ffmpeg_threads` when the
+    admin pinned one (>0), else reserve a core for foreground work (max(1, cpu_count - 1)). Shared by
+    the ffmpeg `-threads` flag and the child's `OMP_NUM_THREADS` so both honour the same budget."""
+    n = settings.download_ffmpeg_threads
+    if n > 0:
+        return n
+    return max(1, (os.cpu_count() or 2) - 1)
+
+
 class EditAborted(Exception):
     """Raised by run_ffmpeg when the job was paused/canceled mid-encode."""
 
@@ -510,11 +523,12 @@ def browser_transcode_flags(
 
 
 def build_browser_safe_cmd(
-    src: Path, dest: Path, reencode_video: bool, reencode_audio: bool
+    src: Path, dest: Path, reencode_video: bool, reencode_audio: bool, threads: int | None = None
 ) -> list[str]:
     """ffmpeg command that lands `src` as a browser-safe mp4 (faststart): re-encode only the
     stream(s) that need it (libx264 / aac), stream-copy the rest, always into mp4. `-progress
-    pipe:1` streams machine-readable progress for run_ffmpeg."""
+    pipe:1` streams machine-readable progress for run_ffmpeg. `threads` caps the encoder so a
+    re-encode can't claim every core on the shared box (see the worker-child de-prioritization)."""
     cmd = [
         "ffmpeg",
         "-y",
@@ -534,6 +548,8 @@ def build_browser_safe_cmd(
         else ["-c:v", "copy"]
     )
     cmd += ["-c:a", "aac", "-b:a", "192k"] if reencode_audio else ["-c:a", "copy"]
+    if threads and threads > 0:
+        cmd += ["-threads", str(threads)]
     cmd += ["-movflags", "+faststart", str(dest)]
     return cmd
 
diff --git a/backend/app/downloads/errors.py b/backend/app/downloads/errors.py
index 68947a8a..0c883bfa 100644
--- a/backend/app/downloads/errors.py
+++ b/backend/app/downloads/errors.py
@@ -24,6 +24,16 @@ _RULES: tuple[tuple[str, str], ...] = (
     # Some extractors (e.g. Reddit) gate content behind a logged-in account: "Account
     # authentication is required." — an auth wall we deliberately don't cross, not a transient error.
     ("authentication is required", "login_required"),
+    # Closed-group / private-post auth walls (Facebook groups, Instagram private, …). yt-dlp phrases
+    # these many ways; the common thread is "you must be signed in" or its own suggestion to supply
+    # cookies. Treating the cookie suggestion as an auth signal is what makes a login-walled link read
+    # honestly (and stay non-retryable) when the acting user has no usable cookie for that host.
+    ("log in to", "login_required"),
+    ("login required", "login_required"),
+    ("you must be logged in", "login_required"),
+    ("only available for registered users", "login_required"),
+    ("use --cookies", "login_required"),
+    ("--cookies-from-browser", "login_required"),
     # Geo-restricted.
     ("in your country", "geo_blocked"),
     ("available in your location", "geo_blocked"),
@@ -42,6 +52,10 @@ _RULES: tuple[tuple[str, str], ...] = (
     ("account associated with this video has been terminated", "unavailable"),
     ("this video is not available", "unavailable"),
     ("this video has been removed", "unavailable"),
+    # yt-dlp doesn't support this site at all ("Unsupported URL: …") — distinct from an auth wall on a
+    # supported site. Non-retryable: no cookie or retry can make an unsupported host downloadable.
+    ("unsupported url", "unsupported_host"),
+    ("no video could be found in this url", "unsupported_host"),
     # DRM.
     ("drm protected", "drm"),
     ("drm-protected", "drm"),
diff --git a/backend/app/downloads/formats.py b/backend/app/downloads/formats.py
index 47a5090b..5ad7dce6 100644
--- a/backend/app/downloads/formats.py
+++ b/backend/app/downloads/formats.py
@@ -98,12 +98,17 @@ def build_ydl_opts(
     outtmpl: str,
     progress_hook,
     extra: dict | None = None,
+    cookiefile: str | None = None,
 ) -> dict:
     """Assemble yt-dlp options for a single download to `outtmpl` (a full path template).
 
     `extra` is source-specific options contributed by the SourceAdapter (e.g. YouTube's
     `extractor_args` with the player-client set + bgutil PO-token base_url); it is merged onto the
-    generic options below. Everything here is source-agnostic."""
+    generic options below. Everything here is source-agnostic.
+
+    `cookiefile` is the acting user's per-host cookies.txt path (S6), attached ONLY by the worker
+    after the host-scope + private-asset gate in `downloads.credentials`. It is set AFTER the `extra`
+    merge so a source adapter can never clobber it."""
     s = normalize(spec)
     h = s["max_height"]
     opts: dict = {
@@ -132,6 +137,11 @@ def build_ydl_opts(
     if extra:
         opts.update(extra)
 
+    # The acting user's per-host cookie (S6). AFTER the adapter merge on purpose: an adapter must not
+    # be able to override the authenticated session the worker resolved for this specific user.
+    if cookiefile:
+        opts["cookiefile"] = cookiefile
+
     if s["mode"] == "a":
         opts["format"] = "bestaudio/best"
         opts["postprocessors"].append(
diff --git a/backend/app/downloads/gc.py b/backend/app/downloads/gc.py
index 9ce499af..a640c22e 100644
--- a/backend/app/downloads/gc.py
+++ b/backend/app/downloads/gc.py
@@ -86,11 +86,25 @@ def _sweep_orphan_files(db: Session, download_root: str) -> int:
 
     cutoff = time.time() - _ORPHAN_MIN_AGE_S
     removed = 0
+    # Throttle the whole-tree walk: this runs in the API process, and an unpaced rglob+stat over a
+    # large download root monopolizes disk IO long enough to make web requests + playback stutter.
+    # Yield the IO channel after every `batch` files touched so foreground work interleaves.
+    pause_s = max(0, settings.download_gc_sweep_pause_ms) / 1000.0
+    batch = max(1, settings.download_gc_sweep_batch)
+    touched = 0
+
+    def _throttle() -> None:
+        nonlocal touched
+        touched += 1
+        if pause_s and touched % batch == 0:
+            time.sleep(pause_s)
+
     for entry in root.iterdir():
         if entry.name.startswith("."):  # .edits / .plex-* / .staging / .storyboards + any dotfile
             continue
         files = [entry] if entry.is_file() else [p for p in entry.rglob("*") if p.is_file()]
         for f in files:
+            _throttle()
             try:
                 if f.resolve() in keep or f.stat().st_mtime > cutoff:
                     continue
diff --git a/backend/app/downloads/links.py b/backend/app/downloads/links.py
index 38dc2038..0344bf17 100644
--- a/backend/app/downloads/links.py
+++ b/backend/app/downloads/links.py
@@ -83,6 +83,10 @@ def public_meta(
     from app.downloads import service
 
     source_url = service.reference_url(job, asset)
+    # Audio-only assets (mp3 rips, extracted audio) carry a codec but no video stream. WebKit (all
+    # iOS browsers) refuses to play them inside a <video> element, so the watch page must render a
+    # native <audio> instead — flag it here from the recorded codecs (a real video always has vcodec).
+    is_audio = asset.vcodec is None and asset.acodec is not None
     return {
         "title": (job and job.display_name) or asset.title,
         "uploader": (job and job.display_uploader) or asset.uploader,
@@ -92,6 +96,7 @@ def public_meta(
         "duration_s": asset.duration_s,
         "width": asset.width,
         "height": asset.height,
+        "audio": is_audio,
         "allow_download": link.allow_download,
         "file_url": file_url,
         "poster_url": poster_url,
diff --git a/backend/app/downloads/pipeline.py b/backend/app/downloads/pipeline.py
index e0ed42a1..be55c786 100644
--- a/backend/app/downloads/pipeline.py
+++ b/backend/app/downloads/pipeline.py
@@ -8,13 +8,14 @@ from datetime import date, datetime, timedelta, timezone
 from pathlib import Path
 
 import yt_dlp
+from sqlalchemy import select
 
 from app import sysconfig
 from app.config import settings
 from app.db import SessionLocal
 from app.downloads import edit as editmod
 from app.downloads import errors as dlerrors
-from app.downloads import adapters, formats, service, storage
+from app.downloads import adapters, credentials, formats, service, storage
 from app.downloads.jobdb import (
     _edit_staging_dir,
     _job_status,
@@ -304,10 +305,13 @@ def _media_info(info: dict) -> dict:
     return {**info, **chosen}
 
 
-def _extract_with_fallback(job_id, asset_id, url, spec, staging, adapter):
+def _extract_with_fallback(job_id, asset_id, url, spec, staging, adapter, cookiefile=None):
     """Download via yt-dlp using the source `adapter`'s client sets + options. For YouTube the adapter
     supplies the primary player-client set then a POT-backed fallback (retried on a bot/DRM/format
-    error); other sources make a single attempt. Returns the info dict of the successful attempt."""
+    error); other sources make a single attempt. Returns the info dict of the successful attempt.
+
+    `cookiefile` (S6) is the acting user's per-host cookies.txt, already gated by
+    `credentials.resolve_cookiefile`; passed to every attempt so the fallback stays authenticated."""
     client_sets = adapter.client_sets()
     outtmpl = str(staging / "%(id)s.%(ext)s")
     last_exc: Exception | None = None
@@ -317,6 +321,7 @@ def _extract_with_fallback(job_id, asset_id, url, spec, staging, adapter):
             outtmpl,
             _make_progress_hook(job_id, asset_id),
             extra=adapter.ydl_opts_extra(clients),
+            cookiefile=cookiefile,
         )
         opts["postprocessor_hooks"] = [_make_pp_hook(job_id)]
         _touch_job(job_id)  # stall-watchdog heartbeat: extraction writes no progress, so a slow /
@@ -381,7 +386,9 @@ def _ensure_browser_playable(job_id: int, produced: Path, spec: dict, info: dict
     # re-encode still shows real progress instead of a frozen 0%.
     total_s = int(info["duration"]) if info.get("duration") else editmod.probe_duration(produced)
     editmod.run_ffmpeg(
-        editmod.build_browser_safe_cmd(produced, dest, reenc_v, reenc_a),
+        editmod.build_browser_safe_cmd(
+            produced, dest, reenc_v, reenc_a, threads=editmod.encode_thread_cap()
+        ),
         total_s=total_s,
         on_progress=_ffmpeg_progress_writer(job_id, "processing"),
         should_cancel=_cancel_poll(job_id),
@@ -395,8 +402,11 @@ def _ensure_browser_playable(job_id: int, produced: Path, spec: dict, info: dict
     return dest
 
 
-def _download(job_id: int, asset_id: int, source_kind: str, source_ref: str, spec: dict) -> None:
+def _download(
+    job_id: int, asset_id: int, source_kind: str, source_ref: str, spec: dict, user_id: int
+) -> None:
     staging = _staging_dir(asset_id)
+    cookiefile: Path | None = None
     try:
         if staging.exists():
             shutil.rmtree(staging, ignore_errors=True)
@@ -405,7 +415,27 @@ def _download(job_id: int, asset_id: int, source_kind: str, source_ref: str, spe
         _set_job(job_id, phase="downloading", progress=0)
         adapter = adapters.resolve_adapter(source_kind, source_ref)
         url = service.source_url(source_kind, source_ref)
-        info = _media_info(_extract_with_fallback(job_id, asset_id, url, spec, staging, adapter))
+        # S6: attach the acting user's per-host cookie ONLY when it's host-scoped to this URL AND
+        # this asset is that user's private, salted identity (the double gate lives in credentials).
+        with SessionLocal() as cdb:
+            asset_sig = cdb.execute(
+                select(MediaAsset.format_sig).where(MediaAsset.id == asset_id)
+            ).scalar_one_or_none()
+            if asset_sig is not None:
+                cookiefile = credentials.resolve_cookiefile(
+                    cdb, user_id, url, asset_sig, spec, staging
+                )
+        info = _media_info(
+            _extract_with_fallback(
+                job_id,
+                asset_id,
+                url,
+                spec,
+                staging,
+                adapter,
+                cookiefile=str(cookiefile) if cookiefile else None,
+            )
+        )
         # Re-resolve now that yt-dlp's info reveals the true source (a bare/redirected url only
         # identifies post-extraction) so the .nfo uniqueid type + uploader_url agree on one source.
         adapter = adapters.resolve_adapter(source_kind, source_ref, info)
@@ -515,6 +545,10 @@ def _download(job_id: int, asset_id: int, source_kind: str, source_ref: str, spe
             db.commit()
         log.warning("job %s failed: %s", job_id, msg)
     finally:
+        # Unlink the decrypted cookie file explicitly (the rmtree below removes the whole staging
+        # tree too, but never leave a plaintext session cookie on disk a moment longer than needed).
+        if cookiefile is not None:
+            cookiefile.unlink(missing_ok=True)
         shutil.rmtree(staging, ignore_errors=True)
 
 
diff --git a/backend/app/downloads/runner.py b/backend/app/downloads/runner.py
index c049208b..8b1d777a 100644
--- a/backend/app/downloads/runner.py
+++ b/backend/app/downloads/runner.py
@@ -12,16 +12,48 @@ progress hook), then exits 0. An unexpected crash exits non-zero and the parent
 """
 
 import logging
+import os
+import shutil
+import subprocess
 import sys
 
+from app.config import settings
 from app.db import SessionLocal
+from app.downloads import edit as editmod
 from app.models import DownloadJob
 
 log = logging.getLogger("siftlode.worker.runner")
 
 
+def _deprioritize() -> None:
+    """Lower THIS child's CPU + IO priority so its yt-dlp/ffmpeg work — every descendant inherits the
+    handicap — can never starve the co-located API, Postgres, or Plex playback. Run once at startup,
+    before any heavy work. Every step is best-effort: a failed renice must never fail a download."""
+    nice_inc = settings.download_child_nice
+    if nice_inc > 0:
+        try:
+            os.nice(nice_inc)  # relative increment; the child starts at the worker's nice (0)
+        except OSError as exc:
+            log.info("could not renice download child: %s", str(exc)[:120])
+    cls = settings.download_child_ionice_class
+    if cls > 0 and shutil.which("ionice"):  # util-linux; absent on some minimal images
+        try:
+            subprocess.run(
+                ["ionice", "-c", str(cls), "-p", str(os.getpid())],
+                capture_output=True,
+                timeout=5,
+                check=False,
+            )
+        except (OSError, subprocess.SubprocessError) as exc:
+            log.info("could not ionice download child: %s", str(exc)[:120])
+    # Cap OpenMP-parallel ffmpeg filters to the same budget as the encoder threads, so a filter
+    # graph can't fan out across every core behind ffmpeg's own `-threads` cap.
+    os.environ.setdefault("OMP_NUM_THREADS", str(editmod.encode_thread_cap()))
+
+
 def main() -> None:
     logging.basicConfig(level=logging.INFO)
+    _deprioritize()
     if len(sys.argv) != 2:
         print("usage: python -m app.downloads.runner <job_id>", file=sys.stderr)
         raise SystemExit(2)
@@ -35,6 +67,7 @@ def main() -> None:
         job_kind = job.job_kind
         spec = dict(job.profile_snapshot or {})
         source_kind, source_ref = job.source_kind, job.source_ref
+        user_id = job.user_id
     if asset_id is None:
         return
 
@@ -45,7 +78,7 @@ def main() -> None:
     if job_kind == "edit":
         _process_edit(job_id, asset_id)
     else:
-        _download(job_id, asset_id, source_kind, source_ref, spec)
+        _download(job_id, asset_id, source_kind, source_ref, spec, user_id)
 
 
 if __name__ == "__main__":
diff --git a/backend/app/downloads/service.py b/backend/app/downloads/service.py
index 71139fef..92a39535 100644
--- a/backend/app/downloads/service.py
+++ b/backend/app/downloads/service.py
@@ -15,6 +15,7 @@ from sqlalchemy.orm import Session
 
 from app.config import settings
 from app.downloads import adapters
+from app.downloads import credentials
 from app.downloads import edit as editmod
 from app.downloads import formats, quota, storage
 from app.downloads.status import HOLDING, JobStatus
@@ -155,7 +156,12 @@ def enqueue(
     Raises quota.QuotaExceeded if the user is at their job-count or footprint cap."""
     quota.check_enqueue(db, user_id)
     spec = formats.normalize(spec)
-    sig = formats.format_sig(spec)
+    # If this user has a stored cookie for the download's host, salt the cache signature with their
+    # id so the cookie-authed result is a PRIVATE asset — never dedup-shared with, or cache-served
+    # to, another user (the load-bearing privacy fix; the worker attaches the cookie to exactly this
+    # salted identity). A normal download (no matching cookie) gets salt=None and shares as before.
+    salt = credentials.salt_for_enqueue(db, user_id, source_url(source_kind, source_ref))
+    sig = formats.format_sig(spec, salt=salt)
     asset = get_or_create_asset(db, source_kind, source_ref, sig)
     # Show a real title + thumbnail the moment it's queued (feed downloads are catalog videos).
     populate_from_catalog(db, asset)
diff --git a/backend/app/models.py b/backend/app/models.py
index 27e19064..f79cb013 100644
--- a/backend/app/models.py
+++ b/backend/app/models.py
@@ -551,6 +551,10 @@ class Playlist(Base, TimestampMixin, UpdatedAtMixin):
     # SHA-256 of name + ordered video ids as last synced from / pushed to YouTube; the
     # baseline `dirty` is derived against. NULL = no baseline yet (treated as dirty).
     synced_fingerprint: Mapped[str | None] = mapped_column(String)
+    # JSON list of the ordered video ids as of the last sync — the 3-way-merge baseline the
+    # two-way YouTube sync classifies each item against (local-add vs YT-add vs real delete).
+    # NULL = no baseline yet; the first merge then does a lossless union (never deletes).
+    synced_video_ids: Mapped[str | None] = mapped_column(Text)
     position: Mapped[int] = mapped_column(Integer, default=0, server_default="0")
 
     items: Mapped[list["PlaylistItem"]] = relationship(
@@ -867,6 +871,35 @@ class DownloadLink(Base, TimestampMixin):
     view_count: Mapped[int] = mapped_column(Integer, default=0, server_default="0")
 
 
+class DownloadCredential(Base, TimestampMixin, UpdatedAtMixin):
+    """A user's stored login cookies for ONE auth-walled download host (Facebook, Instagram, …).
+
+    Per-user, per-host: each user pastes their OWN browser cookies.txt (Netscape format) for a host
+    where they're logged in (e.g. a closed Facebook group they belong to), and a download of a
+    yt-dlp-supported, auth-walled link on that host is retried with the ACTING user's cookie. The
+    blob is Fernet-encrypted at rest (`cookies_enc`) and is NEVER returned by any API/log/audit —
+    the routes are write-only (host + updated_at only). See `downloads.credentials`.
+
+    Two load-bearing security controls live around this row, NOT on it:
+      * **cache privacy** — a cookie-authed download SALTS its MediaAsset `format_sig` with the
+        user id, so the authed asset is private to that user and can never be cache-served to
+        another user (`downloads.credentials.cookie_salt`).
+      * **host scope (SEC-12)** — a cookie is attached ONLY when the download URL's netloc matches
+        this row's `host_key` (`downloads.credentials.host_key_for_url`), so an arbitrary `?url=`
+        can't exfiltrate the user's session to a look-alike host.
+
+    `host_key` is a short canonical key from the supported-host registry (e.g. "facebook"), not the
+    raw netloc — one credential can cover several domains of the same site (facebook.com/fb.watch)."""
+
+    __tablename__ = "download_credentials"
+    __table_args__ = (UniqueConstraint("user_id", "host_key", name="uq_download_credential"),)
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    user_id: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"), index=True)
+    host_key: Mapped[str] = mapped_column(String(64))
+    cookies_enc: Mapped[str] = mapped_column(Text)  # Fernet ciphertext — never returned/logged
+
+
 class MessageKey(Base, TimestampMixin):
     """A user's end-to-end-encryption key material for direct messaging (phase 2).
 
diff --git a/backend/app/plex/watch_sync.py b/backend/app/plex/watch_sync.py
index 85ef6399..0091b9db 100644
--- a/backend/app/plex/watch_sync.py
+++ b/backend/app/plex/watch_sync.py
@@ -439,15 +439,89 @@ def _repush_dirty(db: Session, plex: PlexClient, link: PlexLink) -> int:
     return n
 
 
+# How many stale in-progress movies we probe per incremental run (one metadata() call each). The set
+# is normally tiny (movies started but no longer on Plex's Continue-Watching); the daily full reconcile
+# is the backstop for anything beyond the cap. Most-recently-touched are probed first.
+_MOVIE_PROBE_CAP = 100
+
+
+def _reconcile_inprogress_movies(
+    db: Session, plex: PlexClient, link: PlexLink, on_deck_ids: set[int]
+) -> dict[str, int]:
+    """Close the movie "stuck in progress" gap that the history + onDeck feeds structurally miss.
+
+    When a movie is finished on Plex (often by scrubbing through the end credits) Plex sets
+    ``viewCount>0``, clears ``viewOffset`` and DROPS the movie from Continue-Watching. So onDeck no
+    longer lists it (and onDeck can only ever ADD an in-progress row, never resolve one to watched —
+    see `_pull_apply`), and a scrub-to-end frequently emits no `/status/sessions/history` session row,
+    so the history feed misses it too. The stale local ``status='new', position>0`` row then survives
+    until the *daily* full reconcile. Movies are hit hardest: a movie is a single leaf with no
+    episode-rollup to mask the stuck row, and once watched it leaves onDeck entirely.
+
+    Fix: for each local in-progress MOVIE that Plex's onDeck no longer lists, read the item's current
+    per-(token-)account ``viewCount``/``viewOffset`` with one cheap `metadata()` call and apply it via
+    the shared `_pull_apply` — flipping to watched when Plex says so. Returns ``{"watched", "in_progress",
+    "probed"}`` counts. Episodes are intentionally left to onDeck/history (they promote the next episode
+    and aren't affected). Note: like the rest of this job, `metadata()` reads the TOKEN account (the
+    owner) — a HTPC playing under a managed Plex user (accountID>1) is out of scope here as elsewhere."""
+    conds = [
+        PlexState.user_id == link.user_id,
+        PlexState.status.notin_(("watched", "hidden")),
+        PlexState.position_seconds > _PROGRESS_MIN_S,
+        PlexItem.kind == "movie",
+    ]
+    if on_deck_ids:
+        conds.append(PlexState.item_id.notin_(on_deck_ids))
+    rows = db.execute(
+        select(PlexState, PlexItem.rating_key)
+        .join(PlexItem, PlexItem.id == PlexState.item_id)
+        .where(*conds)
+        .order_by(PlexState.progress_updated_at.desc().nullslast())
+        .limit(_MOVIE_PROBE_CAP + 1)
+    ).all()
+    if len(rows) > _MOVIE_PROBE_CAP:
+        rows = rows[:_MOVIE_PROBE_CAP]
+        log.warning(
+            "in-progress movie probe hit the %d cap (user %s); reconcile catches the tail",
+            _MOVIE_PROBE_CAP,
+            link.user_id,
+        )
+    out = {"watched": 0, "in_progress": 0, "probed": 0}
+    for st, rk in rows:
+        try:
+            meta = plex.metadata(rk)
+        except PlexError as e:
+            log.warning(
+                "movie watch-probe failed (user %s, item %s): %s", link.user_id, st.item_id, e
+            )
+            continue
+        out["probed"] += 1
+        if meta is None:
+            continue
+        res = _plex_watch_to_state(
+            meta.get("viewCount"), meta.get("viewOffset"), meta.get("lastViewedAt")
+        )
+        if res is None:
+            # Plex holds no watch signal now (viewCount 0, no offset) — don't clear it here; an un-watch
+            # is the daily reconcile's job, and a Siftlode-only in-progress row must be preserved.
+            continue
+        status, pos, watched_at, prog_at = res
+        ch = _pull_apply(db, link.user_id, st.item_id, status, pos, watched_at or prog_at)
+        if ch:
+            out[ch] += 1
+    return out
+
+
 def run_plex_watch_sync(db: Session) -> dict:
     """Scheduler job (default 15 min) — the cheap incremental pass. Pulls Plex-side changes since the
     high-water mark (watch history + on-deck, filtered to the owner account) into Siftlode, resolving a
-    divergence to Plex (the source of truth — see `_pull_apply`), then re-pushes any still-unsynced
+    divergence to Plex (the source of truth — see `_pull_apply`), reconciles stale in-progress movies
+    Plex has since finished (see `_reconcile_inprogress_movies`), then re-pushes any still-unsynced
     local states. Does NOT propagate un-watches (the history feed only carries positive views) — that's
     `run_plex_watch_reconcile`'s job."""
     if not sysconfig.get_bool(db, "plex_enabled"):
         return {"skipped": "disabled"}
-    total = {"links": 0, "watched": 0, "in_progress": 0, "repushed": 0}
+    total = {"links": 0, "watched": 0, "in_progress": 0, "repushed": 0, "movie_probes": 0}
     for link in _active_sync_links(db):
         total["links"] += 1
         try:
@@ -467,10 +541,14 @@ def run_plex_watch_sync(db: Session) -> dict:
                     if ch:
                         total[ch] += 1
 
+                # onDeck resolves resume-position changes AND tells us which items Plex still considers
+                # in-progress (needed by the movie reconcile below to know which local rows are stale).
+                on_deck_ids: set[int] = set()
                 for row in plex.on_deck():
                     iid = rk_to_id.get(str(row.get("ratingKey") or ""))
                     if iid is None:
                         continue
+                    on_deck_ids.add(iid)
                     off = int(row.get("viewOffset") or 0) // 1000
                     if off < _PROGRESS_MIN_S:
                         continue
@@ -480,6 +558,11 @@ def run_plex_watch_sync(db: Session) -> dict:
                     if ch:
                         total[ch] += 1
 
+                mv = _reconcile_inprogress_movies(db, plex, link, on_deck_ids)
+                total["watched"] += mv["watched"]
+                total["in_progress"] += mv["in_progress"]
+                total["movie_probes"] += mv["probed"]
+
                 total["repushed"] += _repush_dirty(db, plex, link)
                 link.last_watch_sync_at = now
                 db.commit()
diff --git a/backend/app/routes/downloads.py b/backend/app/routes/downloads.py
index 3722f72a..950639c0 100644
--- a/backend/app/routes/downloads.py
+++ b/backend/app/routes/downloads.py
@@ -16,10 +16,11 @@ from pydantic import BaseModel
 from sqlalchemy import func, select
 from sqlalchemy.orm import Session
 
-from app import sysconfig
+from app import audit, sysconfig
 from app.auth import admin_user, require_human
 from app.config import settings
 from app.db import get_db
+from app.downloads import credentials as dlcreds
 from app.downloads import edit as editmod
 from app.downloads import links as linksmod
 from app.downloads import quota, service, storage
@@ -247,6 +248,65 @@ def delete_profile(
     return {"deleted": profile_id}
 
 
+# --- per-user download auth (cookies) ------------------------------------------------------
+# Write-only surface: the stored blob is NEVER returned — only the supported hosts + whether each
+# is configured + when. See downloads.credentials for the host-scope + cache-privacy guarantees.
+
+
+@router.get("/credentials")
+def list_credentials(user: User = Depends(require_human), db: Session = Depends(get_db)) -> dict:
+    return {"hosts": dlcreds.status_list(db, user.id)}
+
+
+class SetCredentialIn(BaseModel):
+    cookies: str
+
+
+@router.put("/credentials/{host_key}")
+def set_credential(
+    host_key: str,
+    body: SetCredentialIn,
+    user: User = Depends(require_human),
+    db: Session = Depends(get_db),
+) -> dict:
+    """Store (encrypted) this user's cookies.txt for one supported host. Write-only: the value is
+    never echoed back, logged, or audited — only the fact that it changed."""
+    if not dlcreds.is_supported_host(host_key):
+        raise HTTPException(status_code=404, detail="Unknown host")
+    try:
+        cleaned = dlcreds.validate_cookies(body.cookies)
+    except dlcreds.InvalidCookies as e:
+        raise HTTPException(status_code=400, detail={"code": "cookies", "reason": str(e)})
+    dlcreds.upsert_credential(db, user.id, host_key, cleaned)
+    audit.record(
+        db,
+        user.id,
+        audit.AuditAction.DOWNLOAD_CREDENTIAL_SET,
+        target_type="download_credential",
+        target_id=host_key,
+    )
+    db.commit()
+    return {"ok": True, "hosts": dlcreds.status_list(db, user.id)}
+
+
+@router.delete("/credentials/{host_key}")
+def delete_credential(
+    host_key: str, user: User = Depends(require_human), db: Session = Depends(get_db)
+) -> dict:
+    if not dlcreds.is_supported_host(host_key):
+        raise HTTPException(status_code=404, detail="Unknown host")
+    if dlcreds.delete_credential(db, user.id, host_key):
+        audit.record(
+            db,
+            user.id,
+            audit.AuditAction.DOWNLOAD_CREDENTIAL_DELETE,
+            target_type="download_credential",
+            target_id=host_key,
+        )
+    db.commit()
+    return {"ok": True, "hosts": dlcreds.status_list(db, user.id)}
+
+
 def _resolve_spec(
     db: Session, user: User, profile_id: int | None, spec: dict | None
 ) -> tuple[dict, int | None]:
diff --git a/backend/app/routes/playlists.py b/backend/app/routes/playlists.py
index 09dcf4c7..397df8fb 100644
--- a/backend/app/routes/playlists.py
+++ b/backend/app/routes/playlists.py
@@ -14,8 +14,8 @@ from app.db import get_db
 from app.feedquery import feed_columns, serialize
 from app.models import Channel, Playlist, PlaylistItem, User, Video, VideoState
 from app.sync.playlists import (
-    plan_push,
-    push_playlist,
+    merge_playlist,
+    plan_merge,
     recompute_dirty,
     repull_playlist,
     sync_user_playlists,
@@ -329,13 +329,13 @@ def push_plan(
     user: User = Depends(require_write_scope),
     db: Session = Depends(get_db),
 ) -> dict:
-    """Dry run: what 'Sync to YouTube' would do + the quota estimate, so the UI can show a
-    confirm. For a linked playlist this reads the live YouTube state (cheap, 1 unit/page).
-    Write scope is required (require_write_scope) — syncing is a YouTube write."""
+    """Dry run: what a two-way 'Sync to YouTube' merge would do (what each side gains/loses) +
+    the quota estimate, so the UI can show a confirm. For a linked playlist this reads the live
+    YouTube state (cheap, 1 unit/page). Write scope is required — syncing is a YouTube write."""
     pl = _pushable(db, user, playlist_id)
     try:
         with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PUSH):
-            plan = plan_push(db, user, pl)
+            plan = plan_merge(db, user, pl)
     except YouTubeError:
         raise HTTPException(status_code=422, detail="Couldn't read the playlist on YouTube.")
     plan["remaining_today"] = quota.remaining_today(db)
@@ -349,14 +349,14 @@ def push(
     user: User = Depends(require_write_scope),
     db: Session = Depends(get_db),
 ) -> dict:
-    """Push a local playlist to YouTube (create it on first push, else reconcile to match
-    local — local wins). Refuses if the estimated quota exceeds what's left today, so a
-    big playlist can't strand half-pushed. Write scope required (require_write_scope)."""
+    """Two-way sync a local playlist with YouTube (create it on first push, else 3-way merge so
+    neither side loses an item). Refuses if the estimated quota exceeds what's left today, so a
+    big sync can't strand half-applied. Write scope required (require_write_scope)."""
     pl = _pushable(db, user, playlist_id)
     try:
         with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PUSH):
             # Read the live playlist ONCE (linked playlists only) and share it with both the
-            # plan and the push, halving read-quota and closing the second read→write TOCTOU
+            # plan and the merge, halving read-quota and closing the second read→write TOCTOU
             # window. A fresh export (no yt_playlist_id) reads nothing.
             live = None
             if pl.yt_playlist_id:
@@ -365,13 +365,13 @@ def push(
                         list(yt.iter_playlist_items_with_ids(pl.yt_playlist_id)),
                         yt.get_playlist_snippet(pl.yt_playlist_id),
                     )
-            plan = plan_push(db, user, pl, live=live)
+            plan = plan_merge(db, user, pl, live=live)
             if plan["units_estimate"] > quota.remaining_today(db):
                 raise HTTPException(
                     status_code=429,
                     detail="Not enough YouTube quota left today for this sync.",
                 )
-            return push_playlist(db, user, pl, live=live)
+            return merge_playlist(db, user, pl, live=live)
     except YouTubeError:
         db.rollback()
         raise HTTPException(status_code=422, detail="YouTube playlist sync failed.")
diff --git a/backend/app/sync/playlists.py b/backend/app/sync/playlists.py
index 8cbb2a5b..d3201b39 100644
--- a/backend/app/sync/playlists.py
+++ b/backend/app/sync/playlists.py
@@ -1,14 +1,16 @@
 """YouTube playlist sync, both directions. Read: mirror each user's own YouTube playlists
-into local `source='youtube'` playlists (kept fresh by the scheduler). Write-back: push local
-edits out to YouTube (`plan_push` / `push_playlist`), behind the optional write scope.
+into local `source='youtube'` playlists (kept fresh by the scheduler). Write-back: two-way sync
+local edits with YouTube (`plan_merge` / `merge_playlist`), behind the optional write scope.
 
 YouTube's special Watch Later / History playlists are not exposed by the Data API and are
 never synced. Videos in a playlist that aren't in our shared catalog yet are fetched and
 ingested (with a stub channel) so the mirror is faithful."""
 
 import hashlib
+import json
 import logging
 from collections.abc import Callable
+from dataclasses import dataclass
 from datetime import datetime, timezone
 from typing import TypeVar
 
@@ -42,6 +44,57 @@ def current_fingerprint(db: Session, pl: Playlist) -> str:
     return fingerprint(pl.name, _local_video_ids(db, pl))
 
 
+def _dump_ids(ids: list[str]) -> str:
+    """Serialize the ordered id list stored as the 3-way-merge baseline (`synced_video_ids`)."""
+    return json.dumps(ids)
+
+
+def _load_baseline(pl: Playlist) -> tuple[list[str] | None, set[str]]:
+    """Parse `synced_video_ids` into (ordered id set, un-ingestable id set). Two on-disk forms:
+    a plain JSON list (legacy / no un-ingestable ids) or `{"ids": [...], "unavailable": [...]}`.
+    `unavailable` = YouTube-only videos that exist on YouTube but the Data API won't return
+    (private/deleted/region-locked), so they never became local rows — tracked so the merge can
+    skip them instead of forever re-classifying them as a YouTube-add (they'd never converge)."""
+    if not pl.synced_video_ids:
+        return None, set()
+    try:
+        val = json.loads(pl.synced_video_ids)
+    except (ValueError, TypeError):
+        return None, set()
+    if isinstance(val, list):
+        return [str(v) for v in val], set()
+    if isinstance(val, dict):
+        raw_ids = val.get("ids")
+        ids = [str(v) for v in raw_ids] if isinstance(raw_ids, list) else None
+        raw_un = val.get("unavailable")
+        un = {str(v) for v in raw_un} if isinstance(raw_un, list) else set()
+        return ids, un
+    return None, set()
+
+
+def parse_baseline(pl: Playlist) -> list[str] | None:
+    """The last-synced ordered id set, or None when no baseline has been recorded yet (a fresh
+    export, or a link created before S7). None makes the first merge union losslessly."""
+    return _load_baseline(pl)[0]
+
+
+def parse_unavailable(pl: Playlist) -> set[str]:
+    """The last-synced set of un-ingestable-but-live YouTube ids (see `_load_baseline`)."""
+    return _load_baseline(pl)[1]
+
+
+def _set_baseline(pl: Playlist, ids: list[str], unavailable: list[str] | None = None) -> None:
+    """Record `ids` as both the hash (drives `dirty`, over the local ids only) and the raw ordered
+    set (drives merge). `unavailable` = un-ingestable-but-live YouTube ids to carry forward so the
+    next merge skips them; stored only when non-empty (keeps the common case a plain list)."""
+    pl.synced_fingerprint = fingerprint(pl.name, ids)
+    if unavailable:
+        pl.synced_video_ids = json.dumps({"ids": ids, "unavailable": list(unavailable)})
+    else:
+        pl.synced_video_ids = _dump_ids(ids)
+    pl.dirty = False
+
+
 def recompute_dirty(db: Session, pl: Playlist) -> None:
     """Set `dirty` by comparing the current state to the synced baseline. Unlinked lists
     (and Watch later) are never dirty; a missing baseline counts as dirty (unknown YT state)."""
@@ -116,12 +169,18 @@ def _replace_items_from_youtube(
         if v in present and v not in seen:
             seen.add(v)
             ordered.append(v)
+    _set_local_items(db, pl, ordered)
+    # After a pull/reset local == YouTube, so this ordered set IS the new merge baseline.
+    _set_baseline(pl, ordered)
+    return ordered
+
+
+def _set_local_items(db: Session, pl: Playlist, ordered: list[str]) -> None:
+    """Authoritatively replace `pl`'s local items with `ordered` (positions 0..n). The caller
+    is responsible for having ensured every id exists in the catalog."""
     db.execute(delete(PlaylistItem).where(PlaylistItem.playlist_id == pl.id))
     for pos, vid in enumerate(ordered):
         db.add(PlaylistItem(playlist_id=pl.id, video_id=vid, position=pos))
-    pl.synced_fingerprint = fingerprint(pl.name, ordered)
-    pl.dirty = False
-    return ordered
 
 
 def sync_user_playlists(db: Session, user: User) -> dict:
@@ -216,22 +275,91 @@ def _reorder_moves(current: list[str], desired: list[str]) -> int:
     return moves
 
 
-def plan_push(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
-    """Compute (without writing) what pushing this local playlist to YouTube would do:
-    create vs update, how many inserts/deletes/reorders, the quota estimate, and whether
-    YouTube has diverged (items present there that local will remove). Costs read units
-    (1/page) for a linked playlist; nothing for a fresh export. `live` = a pre-fetched
-    (current_items, snippet) tuple so the caller can share one read with push_playlist."""
-    desired = _local_video_ids(db, pl)
+@dataclass
+class MergePlan:
+    """The 3-way merge of a linked playlist: what each side gains and loses so neither side
+    loses an item, plus the merged order. `final` is the id set both sides end up with."""
+
+    final: list[str]  # merged order (local order, YouTube-only adds appended)
+    add_to_yt: list[str]  # local-adds to push out to YouTube
+    add_to_local: list[str]  # YouTube-adds to pull into Siftlode
+    remove_from_yt: list[str]  # items deleted locally that are still on YouTube
+    remove_from_local: list[str]  # items deleted on YouTube that are still local
+    reorders: int  # estimated YouTube playlistItems.update moves to reach `final`
+    baseline_known: bool
+
+
+def merge_plan(
+    baseline: list[str] | None,
+    local: list[str],
+    youtube: list[str],
+    unavailable: set[str] | None = None,
+) -> MergePlan:
+    """Pure 3-way merge. Classify each id against the last-synced `baseline`:
+    - in local but not youtube, not deleted → add to YouTube (a local-add)
+    - in youtube but not local, not deleted → add to Siftlode (a YouTube-add)
+    - in baseline but gone from local → a genuine local delete → propagate to YouTube
+    - in baseline but gone from youtube → a genuine YouTube delete → propagate to Siftlode
+    With NO baseline (first-ever merge) we can't tell an add from a delete, so we UNION both
+    sides (never delete) — the lossless default. Merged order: local order first, YouTube-only
+    additions appended in their YouTube order.
+    `unavailable` = known un-ingestable-but-live YouTube ids: dropped from the YouTube side up
+    front so they're never re-classified as a YouTube-add (they can't ingest) nor as a local
+    delete (they were never local) — otherwise such a video would make the playlist never
+    converge. They stay in place on YouTube, untouched by this merge."""
+    un = set(unavailable or ())
+    L = list(dict.fromkeys(local))
+    Y = [v for v in dict.fromkeys(youtube) if v not in un]
+    Lset, Yset = set(L), set(Y)
+    if baseline is None:
+        add_to_yt = [v for v in L if v not in Yset]
+        add_to_local = [v for v in Y if v not in Lset]
+        remove_from_yt: list[str] = []
+        remove_from_local: list[str] = []
+        final = L + [v for v in Y if v not in Lset]
+    else:
+        B = set(baseline)
+        deleted_local = B - Lset  # was synced, removed on our side
+        deleted_yt = B - Yset  # was synced, removed on YouTube's side
+        gone = deleted_local | deleted_yt
+        remove_from_yt = [v for v in Y if v in deleted_local]
+        remove_from_local = [v for v in L if v in deleted_yt]
+        add_to_yt = [v for v in L if v not in Yset and v not in gone]
+        add_to_local = [v for v in Y if v not in Lset and v not in gone]
+        final = [v for v in L if v not in gone] + [v for v in Y if v not in Lset and v not in gone]
+    final_set = set(final)
+    # On YouTube, after membership reconciliation the order is current-kept + inserts appended;
+    # the reorder pass then moves items into `final`.
+    after_membership = [v for v in Y if v in final_set] + add_to_yt
+    reorders = _reorder_moves(after_membership, final)
+    return MergePlan(
+        final=final,
+        add_to_yt=add_to_yt,
+        add_to_local=add_to_local,
+        remove_from_yt=remove_from_yt,
+        remove_from_local=remove_from_local,
+        reorders=reorders,
+        baseline_known=baseline is not None,
+    )
+
+
+def plan_merge(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
+    """Compute (without writing) what a two-way sync of this playlist would do: create vs
+    update, how many items each side gains/loses, reorders, and the quota estimate (YouTube
+    writes only — local edits are free). Reads live YouTube state (1 unit/page) for a linked
+    playlist; nothing for a fresh export. `live` = a pre-fetched (current_items, snippet) tuple
+    so the caller can share one read with merge_playlist."""
+    local = _local_video_ids(db, pl)
     if not pl.yt_playlist_id:
-        ops = 1 + len(desired)  # create the playlist + one insert per video
+        ops = 1 + len(local)  # create the playlist + one insert per video
         return {
             "action": "create",
-            "to_insert": len(desired),
-            "to_delete": 0,
+            "add_yt": len(local),
+            "add_local": 0,
+            "remove_yt": 0,
+            "remove_local": 0,
             "to_reorder": 0,
             "rename": False,
-            "yt_extra": 0,
             "units_estimate": ops * WRITE_UNIT_COST,
         }
     if live is not None:
@@ -241,23 +369,17 @@ def plan_push(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
             current = list(yt.iter_playlist_items_with_ids(pl.yt_playlist_id))
             snippet = yt.get_playlist_snippet(pl.yt_playlist_id)
     rename = bool(snippet) and (snippet.get("title") or "") != pl.name
-    cur_vids = [it["video_id"] for it in current]
-    cur_set = set(cur_vids)
-    desired_set = set(desired)
-    to_insert = [v for v in desired if v not in cur_set]
-    to_delete = [v for v in cur_vids if v not in desired_set]
-    # After membership reconciliation, the YT order is current-minus-deletes plus inserts
-    # appended; the reorder pass then moves items into the desired order.
-    after_membership = [v for v in cur_vids if v in desired_set] + to_insert
-    reorders = _reorder_moves(after_membership, desired)
-    ops = len(to_insert) + len(to_delete) + reorders + (1 if rename else 0)
+    yt_ids = [it["video_id"] for it in current]
+    mp = merge_plan(parse_baseline(pl), local, yt_ids, parse_unavailable(pl))
+    ops = len(mp.add_to_yt) + len(mp.remove_from_yt) + mp.reorders + (1 if rename else 0)
     return {
         "action": "update",
-        "to_insert": len(to_insert),
-        "to_delete": len(to_delete),
-        "to_reorder": reorders,
+        "add_yt": len(mp.add_to_yt),
+        "add_local": len(mp.add_to_local),
+        "remove_yt": len(mp.remove_from_yt),
+        "remove_local": len(mp.remove_from_local),
+        "to_reorder": mp.reorders,
         "rename": rename,
-        "yt_extra": len(to_delete),
         "units_estimate": ops * WRITE_UNIT_COST,
     }
 
@@ -327,13 +449,16 @@ class _Breaker:
         return out
 
 
-def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
-    """Push a local playlist to YouTube: create it if needed, then reconcile membership
-    and order so YouTube matches local (local wins). Marks the playlist clean on success.
+def merge_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
+    """Two-way sync a local playlist with YouTube: create it on first push, else 3-way MERGE
+    both sides so an item added (or deleted) on either side survives — no side wins wholesale.
+    Local edits are applied directly; YouTube edits go through the failure breaker. On full
+    success the merged set becomes the new baseline and the playlist is marked clean; on any
+    YouTube failure the baseline is left untouched so the next sync retries and converges.
     Caller must have checked the write scope and the quota budget. `live` = a pre-fetched
-    (current_items, snippet) tuple (from plan_push's read) to avoid a second live page-through."""
-    desired = _local_video_ids(db, pl)
-    inserted = deleted = reordered = created = 0
+    (current_items, snippet) tuple (from plan_merge's read) to avoid a second page-through."""
+    local = _local_video_ids(db, pl)
+    inserted = deleted = reordered = created = pulled = removed_local = 0
     failures: list[str] = []
     breaker = _Breaker()
     with YouTubeClient(db, user) as yt:
@@ -341,8 +466,8 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
             pl.yt_playlist_id = yt.create_playlist(pl.name)
             db.commit()
             created = 1
-            # Brand-new playlist: just append every video in order.
-            for vid in desired:
+            # Brand-new playlist: just append every local video in order.
+            for vid in local:
                 added = breaker.run(
                     lambda v=vid: yt.add_playlist_item(pl.yt_playlist_id, v),
                     lambda v=vid: failures.append(v),
@@ -350,60 +475,95 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
                 if not isinstance(added, _Failed):
                     inserted += 1
             if not failures:
-                pl.synced_fingerprint = fingerprint(pl.name, desired)
-                pl.dirty = False
+                _set_baseline(pl, local)
             db.commit()
             return {
                 "created": created,
                 "inserted": inserted,
                 "deleted": deleted,
                 "reordered": reordered,
+                "pulled": pulled,
+                "removed_local": removed_local,
                 "failures": failures,
                 "aborted": breaker.tripped,
                 "yt_playlist_id": pl.yt_playlist_id,
             }
 
-        # Existing link: diff against the live YouTube state (reuse plan_push's read if given).
+        # Existing link: 3-way merge against the live YouTube state (reuse plan_merge's read).
         if live is not None:
             current, snippet = live
         else:
             snippet = yt.get_playlist_snippet(pl.yt_playlist_id)
             current = list(yt.iter_playlist_items_with_ids(pl.yt_playlist_id))
+        item_id_by_vid = {it["video_id"]: it["item_id"] for it in current}
+        yt_ids = [it["video_id"] for it in current]
+        # Re-attempt ingestion of ids previously marked un-ingestable that are still on YouTube: a
+        # video that has since become available (e.g. private→public) should re-enter the merge, not
+        # stay skipped forever. Those that still won't ingest remain filtered (`effective_unavailable`).
+        prev_unavailable = parse_unavailable(pl)
+        still_listed = [v for v in yt_ids if v in prev_unavailable]
+        if still_listed:
+            _ensure_videos(db, yt, still_listed)
+            healed = set(
+                db.execute(select(Video.id).where(Video.id.in_(still_listed))).scalars().all()
+            )
+            effective_unavailable = prev_unavailable - healed
+        else:
+            effective_unavailable = prev_unavailable
+        mp = merge_plan(parse_baseline(pl), local, yt_ids, effective_unavailable)
+
+        # --- Siftlode side (free, direct): pull YouTube-adds in, drop YouTube-deletes, land the
+        # merged order authoritatively. Do this first so local reflects the merge even if a later
+        # YouTube write trips the breaker.
+        _ensure_videos(db, yt, mp.add_to_local)
+        # Only the ids that actually exist in the catalog can become local rows (a private/deleted
+        # YouTube video won't ingest); keep the merged order, dropping any that didn't materialize.
+        present = (
+            set(db.execute(select(Video.id).where(Video.id.in_(mp.final))).scalars().all())
+            if mp.final
+            else set()
+        )
+        final_local = [v for v in mp.final if v in present]
+        pulled = len([v for v in mp.add_to_local if v in present])
+        removed_local = len(mp.remove_from_local)
+        _set_local_items(db, pl, final_local)
+        db.commit()
+
+        # --- YouTube side (metered, breaker-guarded).
         if snippet is not None and (snippet.get("title") or "") != pl.name:
             try:
                 yt.update_playlist_title(pl.yt_playlist_id, pl.name)
             except YouTubeError:
                 failures.append("title")
-        item_id_by_vid = {it["video_id"]: it["item_id"] for it in current}
-        cur_vids = [it["video_id"] for it in current]
-        desired_set = set(desired)
-        # Delete extras (local is authoritative).
-        for it in current:
-            if it["video_id"] not in desired_set:
-                dropped = breaker.run(
-                    lambda i=it: yt.delete_playlist_item(i["item_id"]),
-                    lambda i=it: failures.append(i["video_id"]),
-                )
-                if not isinstance(dropped, _Failed):
-                    deleted += 1
-        # Insert missing (append; the reorder pass below fixes positions).
-        cur_set = set(cur_vids)
-        for vid in desired:
-            if vid not in cur_set:
-                item_id = breaker.run(
-                    lambda v=vid: yt.add_playlist_item(pl.yt_playlist_id, v),
-                    lambda v=vid: failures.append(v),
-                )
-                if not isinstance(item_id, _Failed):
-                    item_id_by_vid[vid] = item_id
-                    inserted += 1
-        # Reorder to match `desired` via insertion-sort moves over a local model.
-        model = [v for v in cur_vids if v in desired_set] + [
-            v for v in desired if v not in cur_set and v in item_id_by_vid
+        # Propagate genuine local deletes.
+        for vid in mp.remove_from_yt:
+            item_id = item_id_by_vid.get(vid)
+            if item_id is None:
+                continue
+            dropped = breaker.run(
+                lambda i=item_id: yt.delete_playlist_item(i),
+                lambda v=vid: failures.append(v),
+            )
+            if not isinstance(dropped, _Failed):
+                deleted += 1
+                item_id_by_vid.pop(vid, None)
+        # Insert local-adds (append; the reorder pass below fixes positions).
+        for vid in mp.add_to_yt:
+            item_id = breaker.run(
+                lambda v=vid: yt.add_playlist_item(pl.yt_playlist_id, v),
+                lambda v=vid: failures.append(v),
+            )
+            if not isinstance(item_id, _Failed):
+                item_id_by_vid[vid] = item_id
+                inserted += 1
+        # Reorder YouTube to match `final` via insertion-sort moves over a local model.
+        final_set = set(mp.final)
+        model = [v for v in yt_ids if v in final_set and v in item_id_by_vid] + [
+            v for v in mp.add_to_yt if v in item_id_by_vid
         ]
-        for i, want in enumerate(desired):
+        for i, want in enumerate(mp.final):
             if want not in item_id_by_vid:
-                continue  # failed insert; skip
+                continue  # deleted-from-YT or failed insert; skip
             if i < len(model) and model[i] == want:
                 continue
             moved = breaker.run(
@@ -419,14 +579,23 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
             model.pop(j)
             model.insert(i, want)
         if not failures:
-            pl.synced_fingerprint = fingerprint(pl.name, desired)
-            pl.dirty = False
+            # Carry forward un-ingestable-but-live YouTube ids so the next merge skips them
+            # (see merge_plan): this run's failed pulls, plus prior ones still on YouTube, minus
+            # any that have since left YouTube. Recomputed each success → self-healing.
+            yt_id_set = set(yt_ids)
+            unavailable = sorted(
+                (effective_unavailable | {v for v in mp.add_to_local if v not in present})
+                & yt_id_set
+            )
+            _set_baseline(pl, final_local, unavailable)
         db.commit()
     return {
         "created": created,
         "inserted": inserted,
         "deleted": deleted,
         "reordered": reordered,
+        "pulled": pulled,
+        "removed_local": removed_local,
         "failures": failures,
         "aborted": breaker.tripped,
         "yt_playlist_id": pl.yt_playlist_id,
diff --git a/backend/tests/test_download_credentials.py b/backend/tests/test_download_credentials.py
new file mode 100644
index 00000000..48e4f42c
--- /dev/null
+++ b/backend/tests/test_download_credentials.py
@@ -0,0 +1,257 @@
+"""S6 — per-user, per-host download cookies: host-scope, cache privacy, encryption, write-only API.
+
+The two load-bearing security properties are covered as behaviour, not prose:
+  * **cache privacy** — a cookie-authed download is a PRIVATE, salted asset; a second user without a
+    cookie enqueuing the same URL gets the shared (unsalted) asset and can never be cache-served the
+    authed one, and the worker attaches a cookie ONLY to that private identity.
+  * **host scope (SEC-12)** — a cookie is resolved ONLY for a URL whose netloc the credential owns; a
+    look-alike or unrelated host resolves to no host_key and gets no cookie.
+
+Pure-logic parts run with no DB; the CRUD / enqueue-salting / API parts use the `db`/`as_user` lane.
+"""
+
+import os
+from pathlib import Path
+
+import pytest
+from cryptography.fernet import Fernet
+
+from app import security
+from app.downloads import credentials, formats, service
+from app.models import DownloadCredential, MediaAsset, User
+
+
+@pytest.fixture(autouse=True)
+def _fernet(monkeypatch):
+    # A throwaway key so encrypt/decrypt work without TOKEN_ENCRYPTION_KEY in the env (mirrors
+    # test_security). Autouse: every credential write in this module encrypts.
+    monkeypatch.setattr(security, "_fernet", Fernet(Fernet.generate_key()))
+
+
+COOKIES = "# Netscape HTTP Cookie File\n.facebook.com\tTRUE\t/\tTRUE\t0\tc_user\t123\n"
+
+
+# --- host scope (pure logic) ------------------------------------------------------------------
+
+
+class TestHostKeyForUrl:
+    def test_matches_a_supported_host(self):
+        assert credentials.host_key_for_url("https://www.facebook.com/watch?v=1") == "facebook"
+        assert credentials.host_key_for_url("https://fb.watch/abc/") == "facebook"
+        assert credentials.host_key_for_url("https://x.com/i/status/1") == "twitter"
+
+    def test_look_alike_host_never_matches(self):
+        assert credentials.host_key_for_url("https://myfacebook.com/x") is None
+        assert credentials.host_key_for_url("https://facebook.com.evil.test/x") is None
+
+    def test_unrelated_or_empty_url_has_no_host_key(self):
+        assert credentials.host_key_for_url("https://www.youtube.com/watch?v=abc") is None
+        assert credentials.host_key_for_url("") is None
+        assert credentials.host_key_for_url(None) is None
+
+
+class TestValidateCookies:
+    def test_accepts_a_netscape_file(self):
+        assert credentials.validate_cookies(COOKIES).endswith("\n")
+
+    def test_rejects_empty(self):
+        with pytest.raises(credentials.InvalidCookies):
+            credentials.validate_cookies("   ")
+
+    def test_rejects_non_cookie_paste(self):
+        # Comments only, no tab-separated data line -> not a usable cookies.txt.
+        with pytest.raises(credentials.InvalidCookies):
+            credentials.validate_cookies("just some pasted text\n# a comment")
+
+
+def test_authed_sig_differs_per_user_and_from_the_shared_sig():
+    spec = formats.normalize({})
+    shared = formats.format_sig(spec)
+    a = credentials.authed_format_sig(spec, 1, "facebook")
+    b = credentials.authed_format_sig(spec, 2, "facebook")
+    assert a != shared and b != shared and a != b
+
+
+def test_write_cookiefile_is_private_and_survives_the_retry_wipe(tmp_path):
+    staging = tmp_path / "staging"
+    staging.mkdir()
+    path = credentials._write_cookiefile(COOKIES, staging)
+    assert path.read_text() == COOKIES
+    # 0600 (owner-only) — POSIX perms only (the worker is Linux-only; Windows has no mode bits).
+    if os.name == "posix":
+        assert (path.stat().st_mode & 0o777) == 0o600
+    # It lives in a SUBDIR so the download retry loop (which unlinks files in the staging ROOT) and
+    # _produced_file (which scans the root) never see it.
+    assert path.parent == staging / ".auth"
+    assert not any(p.is_file() for p in staging.iterdir())
+
+
+# --- CRUD + encryption at rest (DB) -----------------------------------------------------------
+
+
+def _user(db, email="cred@example.com") -> User:
+    u = User(email=email)
+    db.add(u)
+    db.flush()
+    return u
+
+
+class TestCrud:
+    def test_upsert_stores_ciphertext_not_plaintext(self, db):
+        u = _user(db)
+        credentials.upsert_credential(db, u.id, "facebook", COOKIES)
+        db.commit()
+        row = db.get(DownloadCredential, 1)
+        assert row.cookies_enc != COOKIES  # encrypted at rest
+        assert COOKIES not in row.cookies_enc
+        assert security.decrypt(row.cookies_enc) == COOKIES
+
+    def test_upsert_replaces_in_place(self, db):
+        u = _user(db)
+        credentials.upsert_credential(db, u.id, "facebook", COOKIES)
+        db.commit()
+        credentials.upsert_credential(db, u.id, "facebook", COOKIES.replace("123", "999"))
+        db.commit()
+        rows = db.query(DownloadCredential).filter_by(user_id=u.id, host_key="facebook").all()
+        assert len(rows) == 1
+        assert "999" in security.decrypt(rows[0].cookies_enc)
+
+    def test_status_list_never_leaks_the_blob(self, db):
+        u = _user(db)
+        credentials.upsert_credential(db, u.id, "facebook", COOKIES)
+        db.commit()
+        status = credentials.status_list(db, u.id)
+        fb = next(h for h in status if h["host_key"] == "facebook")
+        assert fb["configured"] is True and fb["updated_at"] is not None
+        ig = next(h for h in status if h["host_key"] == "instagram")
+        assert ig["configured"] is False and ig["updated_at"] is None
+        # No field anywhere carries the cookie value.
+        assert all(COOKIES not in str(v) for h in status for v in h.values())
+
+    def test_delete(self, db):
+        u = _user(db)
+        credentials.upsert_credential(db, u.id, "facebook", COOKIES)
+        db.commit()
+        assert credentials.delete_credential(db, u.id, "facebook") is True
+        db.commit()
+        assert credentials.get_credential(db, u.id, "facebook") is None
+        assert credentials.delete_credential(db, u.id, "facebook") is False
+
+
+# --- the crux: enqueue salts privately, no cross-user cache serve (DB) -------------------------
+
+FB_URL = "https://www.facebook.com/watch?v=100"
+
+
+class TestCachePrivacy:
+    def test_enqueue_with_a_cookie_creates_a_private_salted_asset(self, db):
+        u = _user(db)
+        credentials.upsert_credential(db, u.id, "facebook", COOKIES)
+        db.commit()
+        job = service.enqueue(db, u.id, "url", FB_URL, {})
+        asset = db.get(MediaAsset, job.asset_id)
+        spec = formats.normalize({})
+        assert asset.format_sig == credentials.authed_format_sig(spec, u.id, "facebook")
+        assert asset.format_sig != formats.format_sig(spec)
+
+    def test_enqueue_without_a_cookie_uses_the_shared_sig(self, db):
+        u = _user(db)
+        job = service.enqueue(db, u.id, "url", FB_URL, {})
+        asset = db.get(MediaAsset, job.asset_id)
+        assert asset.format_sig == formats.format_sig(formats.normalize({}))
+
+    def test_two_users_never_share_the_authed_asset(self, db):
+        a = _user(db, "a@example.com")
+        b = _user(db, "b@example.com")
+        credentials.upsert_credential(db, a.id, "facebook", COOKIES)
+        db.commit()
+        ja = service.enqueue(db, a.id, "url", FB_URL, {})  # A: authed, private
+        jb = service.enqueue(db, b.id, "url", FB_URL, {})  # B: no cookie, shared
+        assert ja.asset_id != jb.asset_id
+        # B's asset is the shared identity — A's private group video can never be served to B.
+        assert db.get(MediaAsset, jb.asset_id).format_sig == formats.format_sig(
+            formats.normalize({})
+        )
+
+
+# --- the worker gate: attach only to the private identity, host-scoped (DB) --------------------
+
+
+class TestResolveCookiefile:
+    def _asset_sig(self, sig: str) -> MediaAsset:
+        return MediaAsset(
+            source_kind="url", source_ref=FB_URL, format_sig=sig, status="downloading"
+        )
+
+    def test_attaches_to_the_private_salted_asset(self, db, tmp_path):
+        u = _user(db)
+        credentials.upsert_credential(db, u.id, "facebook", COOKIES)
+        db.commit()
+        spec = formats.normalize({})
+        salted = credentials.authed_format_sig(spec, u.id, "facebook")
+        path = credentials.resolve_cookiefile(db, u.id, FB_URL, salted, spec, tmp_path)
+        assert path is not None and Path(path).read_text() == COOKIES
+
+    def test_never_attaches_to_a_shared_asset(self, db, tmp_path):
+        # Cookie exists, but the asset is the UNSALTED shared identity -> attaching would poison the
+        # global cache with a private-group video. Must return None.
+        u = _user(db)
+        credentials.upsert_credential(db, u.id, "facebook", COOKIES)
+        db.commit()
+        spec = formats.normalize({})
+        shared = formats.format_sig(spec)
+        assert credentials.resolve_cookiefile(db, u.id, FB_URL, shared, spec, tmp_path) is None
+
+    def test_non_matching_host_gets_no_cookie(self, db, tmp_path):
+        u = _user(db)
+        credentials.upsert_credential(db, u.id, "facebook", COOKIES)
+        db.commit()
+        spec = formats.normalize({})
+        # A YouTube URL has no supported host_key -> no cookie regardless of what's stored.
+        yt = "https://www.youtube.com/watch?v=abc"
+        salted = credentials.authed_format_sig(spec, u.id, "facebook")
+        assert credentials.resolve_cookiefile(db, u.id, yt, salted, spec, tmp_path) is None
+
+    def test_no_credential_gets_no_cookie(self, db, tmp_path):
+        u = _user(db)
+        spec = formats.normalize({})
+        salted = credentials.authed_format_sig(spec, u.id, "facebook")
+        assert credentials.resolve_cookiefile(db, u.id, FB_URL, salted, spec, tmp_path) is None
+
+
+# --- API: write-only, never echoes the blob (DB) ----------------------------------------------
+
+
+class TestApi:
+    def test_put_get_delete_roundtrip_never_returns_the_blob(self, db, as_user):
+        u = _user(db)
+        db.commit()
+        client = as_user(u)
+
+        r = client.put("/api/downloads/credentials/facebook", json={"cookies": COOKIES})
+        assert r.status_code == 200
+        body = r.json()
+        assert body["ok"] is True
+        assert COOKIES not in r.text  # the response must never carry the value
+        fb = next(h for h in body["hosts"] if h["host_key"] == "facebook")
+        assert fb["configured"] is True
+
+        r = client.get("/api/downloads/credentials")
+        assert r.status_code == 200 and COOKIES not in r.text
+
+        r = client.delete("/api/downloads/credentials/facebook")
+        assert r.status_code == 200
+        assert credentials.get_credential(db, u.id, "facebook") is None
+
+    def test_unknown_host_is_404(self, db, as_user):
+        u = _user(db)
+        db.commit()
+        r = as_user(u).put("/api/downloads/credentials/nope", json={"cookies": COOKIES})
+        assert r.status_code == 404
+
+    def test_bad_cookies_are_rejected(self, db, as_user):
+        u = _user(db)
+        db.commit()
+        r = as_user(u).put("/api/downloads/credentials/facebook", json={"cookies": "   "})
+        assert r.status_code == 400
+        assert r.json()["detail"]["code"] == "cookies"
diff --git a/backend/tests/test_download_deprioritize.py b/backend/tests/test_download_deprioritize.py
new file mode 100644
index 00000000..cfd769f1
--- /dev/null
+++ b/backend/tests/test_download_deprioritize.py
@@ -0,0 +1,103 @@
+"""Worker-child resource de-prioritization (S4): the encoder thread cap, the `-threads` flag on the
+browser-safe re-encode, and the child's startup renice/ionice/OMP handicap. These keep a CPU-heavy
+transcode from pegging every core on the box it shares with the API, Postgres, and Plex playback."""
+
+import subprocess
+
+import pytest
+
+from app.config import settings
+from app.downloads import edit as editmod
+from app.downloads import runner
+
+
+class TestEncodeThreadCap:
+    def test_pinned_setting_wins(self, monkeypatch):
+        monkeypatch.setattr(settings, "download_ffmpeg_threads", 3)
+        assert editmod.encode_thread_cap() == 3
+
+    def test_zero_reserves_one_core(self, monkeypatch):
+        monkeypatch.setattr(settings, "download_ffmpeg_threads", 0)
+        monkeypatch.setattr(editmod.os, "cpu_count", lambda: 8)
+        assert editmod.encode_thread_cap() == 7
+
+    def test_single_core_box_never_caps_below_one(self, monkeypatch):
+        monkeypatch.setattr(settings, "download_ffmpeg_threads", 0)
+        monkeypatch.setattr(editmod.os, "cpu_count", lambda: 1)
+        assert editmod.encode_thread_cap() == 1
+
+    def test_unknown_cpu_count_falls_back(self, monkeypatch):
+        monkeypatch.setattr(settings, "download_ffmpeg_threads", 0)
+        monkeypatch.setattr(editmod.os, "cpu_count", lambda: None)
+        assert editmod.encode_thread_cap() == 1  # max(1, 2 - 1)
+
+
+class TestBrowserSafeThreads:
+    def test_threads_flag_added_when_capped(self):
+        cmd = editmod.build_browser_safe_cmd(
+            "/in.mkv", "/out.mp4", reencode_video=True, reencode_audio=False, threads=2
+        )
+        assert "-threads" in cmd
+        assert cmd[cmd.index("-threads") + 1] == "2"
+        # The cap precedes the output file, so it binds the encoder (not input decoding).
+        assert cmd.index("-threads") < cmd.index("/out.mp4")
+
+    @pytest.mark.parametrize("threads", [0, None])
+    def test_no_threads_flag_when_uncapped(self, threads):
+        cmd = editmod.build_browser_safe_cmd(
+            "/in.mkv", "/out.mp4", reencode_video=True, reencode_audio=False, threads=threads
+        )
+        assert "-threads" not in cmd
+
+
+class TestDeprioritize:
+    def test_renices_by_configured_increment(self, monkeypatch):
+        monkeypatch.setattr(settings, "download_child_nice", 12)
+        monkeypatch.setattr(settings, "download_child_ionice_class", 0)  # skip ionice branch
+        calls = []
+        monkeypatch.setattr(runner.os, "nice", lambda n: calls.append(n))
+        monkeypatch.setattr(runner.os, "environ", {})
+        runner._deprioritize()
+        assert calls == [12]
+
+    def test_nice_zero_does_not_renice(self, monkeypatch):
+        monkeypatch.setattr(settings, "download_child_nice", 0)
+        monkeypatch.setattr(settings, "download_child_ionice_class", 0)
+        calls = []
+        monkeypatch.setattr(runner.os, "nice", lambda n: calls.append(n))
+        monkeypatch.setattr(runner.os, "environ", {})
+        runner._deprioritize()
+        assert calls == []
+
+    def test_sets_omp_budget_to_thread_cap(self, monkeypatch):
+        monkeypatch.setattr(settings, "download_child_nice", 0)
+        monkeypatch.setattr(settings, "download_child_ionice_class", 0)
+        monkeypatch.setattr(settings, "download_ffmpeg_threads", 4)
+        env: dict = {}
+        monkeypatch.setattr(runner.os, "environ", env)
+        runner._deprioritize()
+        assert env["OMP_NUM_THREADS"] == "4"
+
+    def test_ionice_invoked_when_binary_present(self, monkeypatch):
+        monkeypatch.setattr(settings, "download_child_nice", 0)
+        monkeypatch.setattr(settings, "download_child_ionice_class", 3)
+        monkeypatch.setattr(runner.os, "environ", {})
+        monkeypatch.setattr(runner.shutil, "which", lambda name: "/usr/bin/ionice")
+        ran = []
+        monkeypatch.setattr(
+            runner.subprocess,
+            "run",
+            lambda cmd, **kw: ran.append(cmd) or subprocess.CompletedProcess(cmd, 0),
+        )
+        runner._deprioritize()
+        assert ran and ran[0][:3] == ["ionice", "-c", "3"]
+
+    def test_ionice_skipped_when_binary_absent(self, monkeypatch):
+        monkeypatch.setattr(settings, "download_child_nice", 0)
+        monkeypatch.setattr(settings, "download_child_ionice_class", 3)
+        monkeypatch.setattr(runner.os, "environ", {})
+        monkeypatch.setattr(runner.shutil, "which", lambda name: None)
+        ran = []
+        monkeypatch.setattr(runner.subprocess, "run", lambda cmd, **kw: ran.append(cmd))
+        runner._deprioritize()
+        assert ran == []
diff --git a/backend/tests/test_download_errors.py b/backend/tests/test_download_errors.py
index f4e4e55e..3614ec55 100644
--- a/backend/tests/test_download_errors.py
+++ b/backend/tests/test_download_errors.py
@@ -14,6 +14,13 @@ def test_real_ytdlp_messages_map_to_categories():
         "ERROR: [youtube] abc: Sign in to confirm you're not a bot": "login_required",
         "ERROR: [youtube] abc: Join this channel to get access to members-only content": "login_required",
         "ERROR: [Reddit] 1vi1x56: Account authentication is required.": "login_required",
+        # Closed-group / private-post auth walls: yt-dlp either says to sign in or suggests cookies.
+        "ERROR: [facebook] 123: You must be logged in to see this post": "login_required",
+        "ERROR: [facebook] 123: Login required. Use --cookies for the authentication.": "login_required",
+        "ERROR: [instagram] abc: This content is only available for registered users": "login_required",
+        "ERROR: [youtube] abc: Sign in to confirm... Use --cookies-from-browser or --cookies": "login_required",
+        # A site yt-dlp has no extractor for at all — distinct from an auth wall, still non-retryable.
+        "ERROR: Unsupported URL: https://example.com/whatever": "unsupported_host",
         "ERROR: [youtube] abc: The uploader has not made this video available in your country": "geo_blocked",
         "ERROR: [youtube] abc: Requested format is not available": "format_unavailable",
         "ERROR: [youtube] abc: Only images are available for download": "format_unavailable",
diff --git a/backend/tests/test_gc.py b/backend/tests/test_gc.py
index 30da6bf2..3b2ac10e 100644
--- a/backend/tests/test_gc.py
+++ b/backend/tests/test_gc.py
@@ -5,6 +5,8 @@ import os
 import time
 from pathlib import Path
 
+from app.config import settings
+from app.downloads import gc as gcmod
 from app.downloads.gc import _sweep_orphan_files
 
 
@@ -69,3 +71,38 @@ def test_sweep_removes_orphans_but_keeps_owned_system_and_fresh(tmp_path: Path):
     assert not (root / "OldChan").exists()
     assert sysfile.exists()  # system tree untouched
     assert fresh.exists()  # too new to sweep
+
+
+def test_sweep_throttles_after_each_batch(tmp_path: Path, monkeypatch):
+    """The whole-tree walk yields the IO channel after every `batch` files so it can't monopolize
+    disk IO in the API process. batch=2 over 5 touched files → 2 pauses (at file 2 and 4)."""
+    root = tmp_path
+    d = root / "Chan"
+    d.mkdir()
+    for i in range(5):
+        (d / f"orphan_{i}.dat").write_bytes(b"x")
+    for f in d.iterdir():
+        _aged(f, 5)  # old enough that every file is "touched" by the walk
+
+    monkeypatch.setattr(settings, "download_gc_sweep_pause_ms", 10)
+    monkeypatch.setattr(settings, "download_gc_sweep_batch", 2)
+    pauses = []
+    monkeypatch.setattr(gcmod.time, "sleep", lambda s: pauses.append(s))
+
+    _sweep_orphan_files(_StubDB([]), str(root))
+
+    assert pauses == [0.01, 0.01]  # 5 files, batch 2 → yield at #2 and #4
+
+
+def test_sweep_no_throttle_when_pause_disabled(tmp_path: Path, monkeypatch):
+    (tmp_path / "Chan").mkdir()
+    f = tmp_path / "Chan" / "orphan.dat"
+    f.write_bytes(b"x")
+    _aged(f, 5)
+    monkeypatch.setattr(settings, "download_gc_sweep_pause_ms", 0)
+    pauses = []
+    monkeypatch.setattr(gcmod.time, "sleep", lambda s: pauses.append(s))
+
+    _sweep_orphan_files(_StubDB([]), str(tmp_path))
+
+    assert pauses == []
diff --git a/backend/tests/test_links.py b/backend/tests/test_links.py
index 57813b8e..8a7aa55b 100644
--- a/backend/tests/test_links.py
+++ b/backend/tests/test_links.py
@@ -4,7 +4,14 @@ token, rides in the file URL (a <video src> can't send a header), and expires.""
 from datetime import datetime, timedelta, timezone
 from types import SimpleNamespace
 
-from app.downloads.links import _GRANT_TTL, check_grant, is_expired, make_grant, new_token
+from app.downloads.links import (
+    _GRANT_TTL,
+    check_grant,
+    is_expired,
+    make_grant,
+    new_token,
+    public_meta,
+)
 
 
 def test_new_token_is_unguessable_and_unique():
@@ -48,6 +55,44 @@ class TestGrant:
             assert check_grant("tok", bad, 0) is False
 
 
+def _asset(**over):
+    base = dict(
+        title="Clip",
+        uploader="Someone",
+        uploader_url=None,
+        duration_s=42,
+        width=1280,
+        height=720,
+        vcodec="h264",
+        acodec="aac",
+        source_webpage_url=None,
+        source_kind="url",
+    )
+    base.update(over)
+    return SimpleNamespace(**base)
+
+
+class TestPublicMetaAudioFlag:
+    """The watch page picks <audio> vs <video> from this flag; WebKit refuses an audio-only mp3 in a
+    <video>, so an audio-only asset (codec present, no video stream) must report audio=True."""
+
+    _link = SimpleNamespace(allow_download=True)
+
+    def test_audio_only_asset_is_flagged_audio(self):
+        asset = _asset(vcodec=None, acodec="mp3", width=None, height=None)
+        meta = public_meta(self._link, None, asset, "/f.mp3")
+        assert meta["audio"] is True
+
+    def test_a_video_asset_is_not_audio(self):
+        meta = public_meta(self._link, None, _asset(), "/f.mp4")
+        assert meta["audio"] is False
+
+    def test_a_silent_video_is_not_audio(self):
+        # Video present but no audio track — still a video player, not the audio bar.
+        meta = public_meta(self._link, None, _asset(acodec=None), "/f.mp4")
+        assert meta["audio"] is False
+
+
 class TestIsExpired:
     def test_none_expiry_never_expires(self):
         assert is_expired(SimpleNamespace(expires_at=None)) is False
diff --git a/backend/tests/test_playlist_merge.py b/backend/tests/test_playlist_merge.py
new file mode 100644
index 00000000..f4061664
--- /dev/null
+++ b/backend/tests/test_playlist_merge.py
@@ -0,0 +1,360 @@
+"""S7 — two-way (3-way) YouTube playlist merge.
+
+The load-bearing property: an item added (or deleted) on EITHER side survives the sync — the old
+push was one-directional (local wins, deletes everything not local from YouTube), so YouTube-side
+adds were lost. Pure-merge classification runs with no DB; the apply path uses the `db` lane with a
+fake YouTube client so both the local rows and the simulated YouTube state can be asserted.
+"""
+
+from datetime import datetime, timezone
+
+import pytest
+
+from app.models import Channel, Playlist, PlaylistItem, Video
+from app.sync import playlists as pl_mod
+from app.sync.playlists import (
+    merge_plan,
+    merge_playlist,
+    parse_baseline,
+    parse_unavailable,
+    plan_merge,
+)
+from app.youtube.client import YouTubeError
+
+# --- pure merge classification (no DB) --------------------------------------------------------
+
+
+class TestMergePlan:
+    def test_no_baseline_unions_losslessly(self):
+        # First-ever merge: can't tell an add from a delete, so keep everything, delete nothing.
+        mp = merge_plan(None, local=["a", "b"], youtube=["b", "c"])
+        assert mp.add_to_yt == ["a"]  # local-only pushed out
+        assert mp.add_to_local == ["c"]  # youtube-only pulled in
+        assert mp.remove_from_yt == [] and mp.remove_from_local == []
+        assert mp.final == ["a", "b", "c"]  # local order, youtube-only appended
+        assert mp.baseline_known is False
+
+    def test_adds_on_both_sides_both_survive(self):
+        # baseline b,c ; local added 'a' ; youtube added 'd' — nothing deleted.
+        mp = merge_plan(["b", "c"], local=["a", "b", "c"], youtube=["b", "c", "d"])
+        assert mp.add_to_yt == ["a"]
+        assert mp.add_to_local == ["d"]
+        assert mp.remove_from_yt == [] and mp.remove_from_local == []
+        assert mp.final == ["a", "b", "c", "d"]
+
+    def test_genuine_local_delete_propagates_to_youtube(self):
+        # baseline a,b,c ; local removed 'b' ; youtube unchanged -> remove b from youtube.
+        mp = merge_plan(["a", "b", "c"], local=["a", "c"], youtube=["a", "b", "c"])
+        assert mp.remove_from_yt == ["b"]
+        assert mp.add_to_yt == [] and mp.add_to_local == []
+        assert mp.final == ["a", "c"]
+
+    def test_genuine_youtube_delete_propagates_to_local(self):
+        # baseline a,b,c ; youtube removed 'b' ; local unchanged -> remove b from local.
+        mp = merge_plan(["a", "b", "c"], local=["a", "b", "c"], youtube=["a", "c"])
+        assert mp.remove_from_local == ["b"]
+        assert mp.final == ["a", "c"]
+
+    def test_delete_on_both_sides_is_gone_not_a_conflict(self):
+        mp = merge_plan(["a", "b", "c"], local=["a", "c"], youtube=["a", "c"])
+        assert mp.remove_from_yt == [] and mp.remove_from_local == []
+        assert mp.final == ["a", "c"]
+
+    def test_a_local_delete_beats_a_stale_youtube_copy(self):
+        # 'b' was synced, deleted locally, still present on youtube -> it must be removed there,
+        # NOT resurrected into local as a youtube-add.
+        mp = merge_plan(["a", "b"], local=["a"], youtube=["a", "b"])
+        assert mp.add_to_local == []
+        assert mp.remove_from_yt == ["b"]
+        assert "b" not in mp.final
+
+
+# --- apply path (db lane + fake YouTube) ------------------------------------------------------
+
+
+class FakeYouTube:
+    """Enough of YouTubeClient for merge_playlist: an in-memory playlist of (item_id, video_id)
+    and the write ops that mutate it. Every catalog video is pre-seeded so _ensure_videos never
+    needs get_videos."""
+
+    def __init__(
+        self,
+        *,
+        title: str,
+        items: list[str],
+        unavailable: set[str] | None = None,
+        fail_add: set[str] | None = None,
+    ):
+        self.title = title
+        self._seq = 0
+        self.items = [(self._new_id(), v) for v in items]
+        # Ids the Data API won't return (private/deleted) — get_videos omits them, so _ensure_videos
+        # can't ingest them. They still live in the playlist (`items`) like a real un-ingestable video.
+        self.unavailable = set(unavailable or ())
+        # Ids whose add_playlist_item raises, to exercise the partial-failure baseline-hold path.
+        self.fail_add = set(fail_add or ())
+
+    def _new_id(self) -> str:
+        self._seq += 1
+        return f"it{self._seq}"
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *exc):
+        return False
+
+    def create_playlist(self, title, description="", privacy="private"):
+        self.title = title
+        return "PLnew"
+
+    def get_playlist_snippet(self, pid):
+        return {"title": self.title}
+
+    def iter_playlist_items_with_ids(self, pid):
+        return [{"item_id": iid, "video_id": v} for iid, v in self.items]
+
+    def get_videos(self, ids):
+        # Mirror the Data API: a minimal ingestable record for each id, EXCEPT the un-ingestable
+        # ones (private/deleted), which are simply absent from the response.
+        return [
+            {
+                "id": v,
+                "snippet": {
+                    "channelId": "UCtest",
+                    "channelTitle": "Test",
+                    "publishedAt": "2020-01-01T00:00:00Z",
+                },
+            }
+            for v in ids
+            if v not in self.unavailable
+        ]
+
+    def update_playlist_title(self, pid, title):
+        self.title = title
+
+    def add_playlist_item(self, pid, vid, position=None):
+        if vid in self.fail_add:
+            raise YouTubeError("simulated add failure")
+        iid = self._new_id()
+        self.items.append((iid, vid))
+        return iid
+
+    def delete_playlist_item(self, item_id):
+        self.items = [(iid, v) for iid, v in self.items if iid != item_id]
+
+    def move_playlist_item(self, item_id, pid, vid, position):
+        row = next((r for r in self.items if r[0] == item_id), None)
+        if row is not None:
+            self.items.remove(row)
+            self.items.insert(position, row)
+
+    @property
+    def video_ids(self) -> list[str]:
+        return [v for _, v in self.items]
+
+
+def _seed_videos(db, ids):
+    db.add(Channel(id="UCtest", title="Test"))
+    now = datetime.now(timezone.utc)
+    for v in ids:
+        db.add(Video(id=v, channel_id="UCtest", published_at=now))
+    db.commit()
+
+
+def _linked_playlist(db, user, *, local_ids, baseline):
+    pl = Playlist(
+        user_id=user.id,
+        name="Mix",
+        source="local",
+        yt_playlist_id="PLyt",
+        synced_video_ids=None if baseline is None else pl_mod._dump_ids(baseline),
+    )
+    db.add(pl)
+    db.flush()
+    for pos, v in enumerate(local_ids):
+        db.add(PlaylistItem(playlist_id=pl.id, video_id=v, position=pos))
+    db.commit()
+    return pl
+
+
+def _local_ids(db, pl):
+    return pl_mod._local_video_ids(db, pl)
+
+
+@pytest.fixture
+def _fake_yt(monkeypatch):
+    holder = {}
+
+    def install(fake):
+        holder["fake"] = fake
+        monkeypatch.setattr(pl_mod, "YouTubeClient", lambda db, user: fake)
+        return fake
+
+    return install
+
+
+def test_adds_on_both_sides_survive_on_both_sides(db, make_user, _fake_yt):
+    user = make_user()
+    _seed_videos(db, ["a", "b", "c", "d"])
+    # baseline b,c ; local added 'a' ; youtube added 'd'
+    pl = _linked_playlist(db, user, local_ids=["a", "b", "c"], baseline=["b", "c"])
+    fake = _fake_yt(FakeYouTube(title="Mix", items=["b", "c", "d"]))
+
+    r = merge_playlist(db, user, pl)
+
+    assert r["failures"] == []
+    assert set(_local_ids(db, pl)) == {"a", "b", "c", "d"}, "youtube-add 'd' pulled into Siftlode"
+    assert set(fake.video_ids) == {"a", "b", "c", "d"}, "local-add 'a' pushed to YouTube"
+    assert r["inserted"] == 1 and r["pulled"] == 1
+    # Baseline advanced to the merged set so a re-sync is a no-op.
+    assert set(parse_baseline(pl)) == {"a", "b", "c", "d"}
+
+
+def test_a_genuine_delete_propagates_without_collateral_loss(db, make_user, _fake_yt):
+    user = make_user()
+    _seed_videos(db, ["a", "b", "c", "d"])
+    # baseline a,b,c ; local removed 'b' ; youtube added 'd' (a real add, must NOT be collateral)
+    pl = _linked_playlist(db, user, local_ids=["a", "c"], baseline=["a", "b", "c"])
+    fake = _fake_yt(FakeYouTube(title="Mix", items=["a", "b", "c", "d"]))
+
+    r = merge_playlist(db, user, pl)
+
+    assert r["failures"] == []
+    assert set(fake.video_ids) == {"a", "c", "d"}, "'b' removed on YouTube, 'd' kept"
+    assert set(_local_ids(db, pl)) == {"a", "c", "d"}, "'d' pulled in, 'b' stays deleted"
+    assert r["deleted"] == 1
+
+
+def test_first_merge_with_no_baseline_never_deletes(db, make_user, _fake_yt):
+    user = make_user()
+    _seed_videos(db, ["a", "b", "c"])
+    pl = _linked_playlist(db, user, local_ids=["a", "b"], baseline=None)
+    fake = _fake_yt(FakeYouTube(title="Mix", items=["b", "c"]))
+
+    r = merge_playlist(db, user, pl)
+
+    assert r["deleted"] == 0 and r["removed_local"] == 0
+    assert set(_local_ids(db, pl)) == {"a", "b", "c"}
+    assert set(fake.video_ids) == {"a", "b", "c"}
+    assert parse_baseline(pl) is not None, "a baseline is recorded after the first union"
+
+
+def test_create_path_appends_and_records_baseline(db, make_user, _fake_yt):
+    user = make_user()
+    _seed_videos(db, ["a", "b"])
+    pl = Playlist(user_id=user.id, name="Fresh", source="local")
+    db.add(pl)
+    db.flush()
+    for pos, v in enumerate(["a", "b"]):
+        db.add(PlaylistItem(playlist_id=pl.id, video_id=v, position=pos))
+    db.commit()
+    _fake_yt(FakeYouTube(title="Fresh", items=[]))
+
+    r = merge_playlist(db, user, pl)
+
+    assert r["created"] == 1 and r["inserted"] == 2
+    assert pl.yt_playlist_id  # created and linked
+    assert parse_baseline(pl) == ["a", "b"]
+
+
+def test_reorder_reaches_the_merged_order_on_youtube(db, make_user, _fake_yt):
+    # Same membership on both sides, but the user reordered locally to c,a,b. The apply-side
+    # insertion-sort must drive YouTube to that exact order — asserted as an ordered list, not a
+    # set, so an off-by-one or a dropped reorder pass can't pass unnoticed.
+    user = make_user()
+    _seed_videos(db, ["a", "b", "c"])
+    pl = _linked_playlist(db, user, local_ids=["c", "a", "b"], baseline=["a", "b", "c"])
+    fake = _fake_yt(FakeYouTube(title="Mix", items=["a", "b", "c"]))
+
+    r = merge_playlist(db, user, pl)
+
+    assert r["failures"] == []
+    assert fake.video_ids == ["c", "a", "b"], "YouTube order matches the merged (local) order"
+    assert r["reordered"] >= 1
+    assert parse_baseline(pl) == ["c", "a", "b"]
+
+
+def test_partial_youtube_failure_holds_the_baseline_for_retry(db, make_user, _fake_yt):
+    # A local-add whose YouTube insert fails must NOT advance the baseline — otherwise the retry
+    # sync would read the un-pushed add as a genuine YouTube-side delete and silently drop it.
+    user = make_user()
+    _seed_videos(db, ["a", "b", "c"])
+    pl = _linked_playlist(db, user, local_ids=["a", "b", "c"], baseline=["b", "c"])
+    fake = _fake_yt(FakeYouTube(title="Mix", items=["b", "c"], fail_add={"a"}))
+
+    r = merge_playlist(db, user, pl)
+
+    assert r["failures"] == ["a"], "the failed local-add is reported"
+    assert "a" not in fake.video_ids, "the add never landed on YouTube"
+    # Baseline is UNCHANGED so the next sync re-classifies 'a' as a local-add and re-pushes it,
+    # rather than treating it as a YouTube delete and removing it locally.
+    assert parse_baseline(pl) == ["b", "c"]
+    assert "a" in set(_local_ids(db, pl)), "'a' stays local for the retry"
+
+
+def test_unavailable_youtube_video_converges(db, make_user, _fake_yt):
+    # A YouTube-only video that can't be ingested (private/deleted) must not make the playlist
+    # loop forever re-classifying it as a pending pull. It is recorded as un-ingestable and skipped
+    # by the next merge; the playlist reports "up to date".
+    user = make_user()
+    _seed_videos(db, ["a", "b"])  # note: 'x' is NOT in the catalog and can't be ingested
+    pl = _linked_playlist(db, user, local_ids=["a", "b"], baseline=["a", "b"])
+    fake = _fake_yt(FakeYouTube(title="Mix", items=["a", "b", "x"], unavailable={"x"}))
+
+    r1 = merge_playlist(db, user, pl)
+
+    assert r1["failures"] == []
+    assert r1["pulled"] == 0, "the un-ingestable video did not become a local row"
+    assert set(_local_ids(db, pl)) == {"a", "b"}
+    assert parse_unavailable(pl) == {"x"}, "'x' recorded as un-ingestable"
+    assert "x" in fake.video_ids, "'x' left untouched on YouTube"
+
+    # Second sync: the preview reports nothing pending (plan_merge filters 'x' via the stored
+    # unavailable set). The sync itself re-attempts ingestion of 'x' (the heal path), but since it
+    # still won't ingest, 'x' stays recorded as unavailable — no perpetual pending pull.
+    plan = plan_merge(db, user, pl)
+    assert plan["add_local"] == 0, "no perpetual pending pull — the playlist is up to date"
+    r2 = merge_playlist(db, user, pl)
+    assert r2["pulled"] == 0 and r2["failures"] == []
+    assert parse_unavailable(pl) == {"x"}
+    assert "x" in fake.video_ids
+
+
+def test_unavailable_video_that_becomes_available_is_pulled_in(db, make_user, _fake_yt):
+    # A video marked un-ingestable (e.g. private) that later becomes available (public) must
+    # re-enter the merge on a normal sync — not stay skipped forever.
+    user = make_user()
+    _seed_videos(db, ["a", "b"])
+    pl = _linked_playlist(db, user, local_ids=["a", "b"], baseline=["a", "b"])
+    fake = _fake_yt(FakeYouTube(title="Mix", items=["a", "b", "x"], unavailable={"x"}))
+
+    merge_playlist(db, user, pl)
+    assert parse_unavailable(pl) == {"x"}, "recorded un-ingestable on the first sync"
+
+    # 'x' becomes available; the next sync re-attempts ingestion and pulls it in.
+    fake.unavailable.clear()
+    r2 = merge_playlist(db, user, pl)
+
+    assert r2["failures"] == []
+    assert "x" in set(_local_ids(db, pl)), "the now-available video is pulled into Siftlode"
+    assert parse_unavailable(pl) == set(), "'x' is no longer marked un-ingestable"
+
+
+def test_unavailable_id_that_leaves_youtube_is_pruned(db, make_user, _fake_yt):
+    # A recorded un-ingestable id that is later removed from the YouTube playlist must be pruned
+    # from the persisted `unavailable` set (the `& yt_id_set` self-heal), not carried forever.
+    user = make_user()
+    _seed_videos(db, ["a", "b"])
+    pl = _linked_playlist(db, user, local_ids=["a", "b"], baseline=["a", "b"])
+    fake = _fake_yt(FakeYouTube(title="Mix", items=["a", "b", "x"], unavailable={"x"}))
+
+    merge_playlist(db, user, pl)
+    assert parse_unavailable(pl) == {"x"}, "recorded un-ingestable on the first sync"
+
+    # 'x' is removed from the YouTube playlist entirely; the next sync must drop it.
+    fake.items = [row for row in fake.items if row[1] != "x"]
+    r2 = merge_playlist(db, user, pl)
+
+    assert r2["failures"] == []
+    assert parse_unavailable(pl) == set(), "'x' pruned once it left YouTube"
diff --git a/backend/tests/test_plex_watch_sync.py b/backend/tests/test_plex_watch_sync.py
index da20bfee..c812d147 100644
--- a/backend/tests/test_plex_watch_sync.py
+++ b/backend/tests/test_plex_watch_sync.py
@@ -9,11 +9,12 @@ Two independent pieces:
 """
 
 from datetime import datetime, timedelta, timezone
+from types import SimpleNamespace
 
 from sqlalchemy import select
 
 from app.models import PlexItem, PlexLibrary, PlexState, User
-from app.plex.watch_sync import _pull_apply
+from app.plex.watch_sync import _pull_apply, _reconcile_inprogress_movies
 from app.routes.plex.playback import (
     _PUSH_THROTTLE_S,
     _resume_push_superseded,
@@ -33,6 +34,16 @@ def _seed_item_and_user(db):
     return item.id, user.id
 
 
+class _FakePlex:
+    """Minimal PlexClient stand-in for the movie-probe tests: metadata(rating_key) → a leaf dict."""
+
+    def __init__(self, by_rk: dict[str, dict]):
+        self._by_rk = by_rk
+
+    def metadata(self, rating_key: str, markers: bool = False):
+        return self._by_rk.get(str(rating_key))
+
+
 def test_pull_plex_wins_over_diverging_synced_local_resume(db):
     """The core S2 change: a SYNCED local row (its state already reached Plex) that later diverges from
     Plex = a real external Plex-side change, so the incremental pull resolves to PLEX's position — Plex
@@ -198,6 +209,130 @@ def test_pull_settles_flag_on_matching_dirty_row(db):
     assert st.synced_to_plex is True, "the dirty flag is settled so it isn't re-pushed endlessly"
 
 
+def test_movie_probe_flips_stale_in_progress_to_watched(db):
+    """S1 core: a local in-progress MOVIE that Plex has since finished (viewCount>0) and dropped from
+    Continue-Watching flips to watched on the incremental pass — without waiting for the daily reconcile.
+    This is the "movie stuck in progress after scrubbing the credits" bug."""
+    item_id, user_id = _seed_item_and_user(db)
+    now = datetime.now(timezone.utc)
+    db.add(
+        PlexState(
+            user_id=user_id,
+            item_id=item_id,
+            status="new",
+            position_seconds=300,
+            progress_updated_at=now,
+            synced_to_plex=True,  # was created by a prior pull; the user then watched it on Plex
+        )
+    )
+    db.flush()
+
+    # Plex now reports the movie watched (viewCount>0, no offset) and it is NOT on the deck.
+    plex = _FakePlex({"m1": {"viewCount": 1, "lastViewedAt": int(now.timestamp())}})
+    out = _reconcile_inprogress_movies(
+        db, plex, SimpleNamespace(user_id=user_id), on_deck_ids=set()
+    )
+    db.flush()
+
+    assert out == {"watched": 1, "in_progress": 0, "probed": 1}
+    st = (
+        db.execute(
+            select(PlexState).where(PlexState.user_id == user_id, PlexState.item_id == item_id)
+        )
+        .scalars()
+        .one()
+    )
+    assert st.status == "watched"
+    assert st.position_seconds == 0
+
+
+def test_movie_probe_skips_ondeck_and_episodes(db):
+    """The probe must NOT touch (a) a movie Plex still lists on the deck — it's legitimately in
+    progress — nor (b) an in-progress EPISODE (episodes ride onDeck/history + rollup). Only stale
+    in-progress movies are probed, and metadata() is called for those alone."""
+    lib = PlexLibrary(plex_key="1", title="Movies", kind="movie", enabled=True)
+    db.add(lib)
+    db.flush()
+    on_deck_movie = PlexItem(rating_key="mDeck", library_id=lib.id, kind="movie", title="On deck")
+    episode = PlexItem(rating_key="e1", library_id=lib.id, kind="episode", title="An episode")
+    db.add_all([on_deck_movie, episode])
+    user = User(email="probe2@example.com")
+    db.add(user)
+    db.flush()
+    now = datetime.now(timezone.utc)
+    for iid in (on_deck_movie.id, episode.id):
+        db.add(
+            PlexState(
+                user_id=user.id,
+                item_id=iid,
+                status="new",
+                position_seconds=300,
+                progress_updated_at=now,
+                synced_to_plex=True,
+            )
+        )
+    db.flush()
+
+    # Plex would report both watched — but neither should be probed at all, so the fake returning
+    # "watched" must have NO effect.
+    plex = _FakePlex(
+        {
+            "mDeck": {"viewCount": 1, "lastViewedAt": int(now.timestamp())},
+            "e1": {"viewCount": 1, "lastViewedAt": int(now.timestamp())},
+        }
+    )
+    out = _reconcile_inprogress_movies(
+        db, plex, SimpleNamespace(user_id=user.id), on_deck_ids={on_deck_movie.id}
+    )
+    db.flush()
+
+    assert out == {"watched": 0, "in_progress": 0, "probed": 0}, "neither item is probed"
+    for iid in (on_deck_movie.id, episode.id):
+        st = (
+            db.execute(
+                select(PlexState).where(PlexState.user_id == user.id, PlexState.item_id == iid)
+            )
+            .scalars()
+            .one()
+        )
+        assert st.status == "new" and st.position_seconds == 300, "left untouched"
+
+
+def test_movie_probe_leaves_row_when_plex_has_no_signal(db):
+    """If Plex reports no watch signal (viewCount 0, no offset) for a probed movie, the incremental pass
+    must NOT clear the local in-progress row — un-watch is the daily reconcile's job, and a Siftlode-only
+    in-progress state must survive."""
+    item_id, user_id = _seed_item_and_user(db)
+    now = datetime.now(timezone.utc)
+    db.add(
+        PlexState(
+            user_id=user_id,
+            item_id=item_id,
+            status="new",
+            position_seconds=300,
+            progress_updated_at=now,
+            synced_to_plex=True,
+        )
+    )
+    db.flush()
+
+    plex = _FakePlex({"m1": {"viewCount": 0}})
+    out = _reconcile_inprogress_movies(
+        db, plex, SimpleNamespace(user_id=user_id), on_deck_ids=set()
+    )
+    db.flush()
+
+    assert out == {"watched": 0, "in_progress": 0, "probed": 1}
+    st = (
+        db.execute(
+            select(PlexState).where(PlexState.user_id == user_id, PlexState.item_id == item_id)
+        )
+        .scalars()
+        .one()
+    )
+    assert st.status == "new" and st.position_seconds == 300, "in-progress row preserved"
+
+
 def test_should_push_resume_scopes_by_user_and_item():
     """The throttle window is keyed by the FULL (user, item) tuple: pushing for one key must not
     suppress a distinct user or a distinct item within the same window."""
diff --git a/frontend/src/components/ChannelPage.tsx b/frontend/src/components/ChannelPage.tsx
index 80ebce67..682388d3 100644
--- a/frontend/src/components/ChannelPage.tsx
+++ b/frontend/src/components/ChannelPage.tsx
@@ -34,6 +34,7 @@ import type { FeedView } from "../lib/feedView";
 import { channelYouTubeUrl, formatViews } from "../lib/format";
 import { ChannelTagsControl } from "./ChannelTags";
 import { useChannelTagActions } from "../lib/useChannelTagActions";
+import { Button } from "./ui/Button";
 
 // A dedicated channel page: an "About"-style header (banner, avatar, stats, subscribe) over the
 // channel's videos (the catalog filtered to this channel). For an un-subscribed channel it
@@ -441,21 +442,18 @@ export default function ChannelPage({
                 it) so a large channel's paced tail reads as visible progress rather than a stuck
                 spinner. */}
               {ch?.subscribed && !ch.backfill_done && !me.is_demo && (
-                <button
+                <Button
+                  variant="outline"
                   onClick={() => !ch.deep_requested && deepBackfill.mutate()}
                   disabled={ch.deep_requested || deepBackfill.isPending}
+                  loading={ch.deep_requested}
+                  icon={<History className="w-4 h-4" />}
                   title={
                     ch.deep_requested
                       ? t("channel.fullHistoryLoadingHint")
                       : t("channel.fullHistory")
                   }
-                  className="inline-flex items-center gap-1.5 px-3 py-2 rounded-lg text-sm border border-border text-muted hover:text-accent hover:border-accent disabled:opacity-60 disabled:hover:text-muted disabled:hover:border-border transition"
                 >
-                  {ch.deep_requested ? (
-                    <Loader2 className="w-4 h-4 animate-spin" />
-                  ) : (
-                    <History className="w-4 h-4" />
-                  )}
                   <span className="hidden sm:inline whitespace-nowrap">
                     {!ch.deep_requested
                       ? t("channel.fullHistory")
@@ -466,7 +464,7 @@ export default function ChannelPage({
                           })
                         : t("channel.fullHistoryLoading")}
                   </span>
-                </button>
+                </Button>
               )}
               {!me.is_demo && (
                 <button
@@ -504,24 +502,24 @@ export default function ChannelPage({
               {!me.is_demo &&
                 !ch?.blocked &&
                 (ch?.subscribed ? (
-                  <button
+                  <Button
+                    variant="outline"
                     onClick={onUnsubscribe}
                     disabled={unsubscribe.isPending}
                     title={t("channel.unsubscribe")}
-                    className="inline-flex items-center gap-1.5 px-3 py-2 rounded-lg text-sm border border-border text-muted hover:text-fg disabled:opacity-50"
+                    icon={<Check className="w-4 h-4" />}
                   >
-                    <Check className="w-4 h-4" />
                     {t("channel.subscribedState")}
-                  </button>
+                  </Button>
                 ) : (
-                  <button
+                  <Button
+                    variant="outline"
                     onClick={() => subscribe.mutate()}
                     disabled={subscribe.isPending}
-                    className="inline-flex items-center gap-1.5 px-3 py-2 rounded-lg text-sm btn-primary disabled:opacity-50"
+                    icon={<Plus className="w-4 h-4" />}
                   >
-                    <Plus className="w-4 h-4" />
                     {t("channel.subscribe")}
-                  </button>
+                  </Button>
                 ))}
               {/* Tagging is a subscription-scoped action (attach_tag → _user_subscription 404s
                   otherwise), and personal tags are for your subscriptions — so only offer it on a
diff --git a/frontend/src/components/ConfirmProvider.tsx b/frontend/src/components/ConfirmProvider.tsx
index 6051ff53..91b5f331 100644
--- a/frontend/src/components/ConfirmProvider.tsx
+++ b/frontend/src/components/ConfirmProvider.tsx
@@ -1,6 +1,7 @@
 import { createContext, useCallback, useContext, useState, type ReactNode } from "react";
 import { useTranslation } from "react-i18next";
 import Modal from "./Modal";
+import { Button } from "./ui/Button";
 
 // App-styled, promise-based replacement for window.confirm. Wrap the tree in
 // <ConfirmProvider> once, then anywhere:  const confirm = useConfirm();
@@ -100,26 +101,25 @@ export function ConfirmProvider({ children }: { children: ReactNode }) {
           <div
             className={multi ? "flex flex-col gap-2 mt-5" : "flex justify-end gap-2 mt-5 flex-wrap"}
           >
-            <button
+            <Button
+              variant="outline"
               autoFocus={anyDanger}
               onClick={() => close(null)}
-              className={`px-4 py-2 rounded-lg text-sm border border-border text-muted hover:text-fg hover:bg-surface transition ${
-                multi ? "w-full order-last" : ""
-              }`}
+              fullWidth={multi}
+              className={multi ? "order-last" : ""}
             >
               {state.opts.cancelLabel ?? t("common.cancel")}
-            </button>
+            </Button>
             {choices.map((c, i) => (
-              <button
+              <Button
                 key={c.id}
+                variant={c.danger ? "danger" : "primary"}
                 autoFocus={!anyDanger && i === choices.length - 1}
                 onClick={() => close(c.id)}
-                className={`px-4 py-2 rounded-lg text-sm font-semibold transition ${
-                  multi ? "w-full" : ""
-                } ${c.danger ? "btn-danger" : "btn-primary"}`}
+                fullWidth={multi}
               >
                 {c.label || t("common.confirm")}
-              </button>
+              </Button>
             ))}
           </div>
         </Modal>
diff --git a/frontend/src/components/DownloadCenter.tsx b/frontend/src/components/DownloadCenter.tsx
index b17c21b7..46e5c9bb 100644
--- a/frontend/src/components/DownloadCenter.tsx
+++ b/frontend/src/components/DownloadCenter.tsx
@@ -31,6 +31,7 @@ import { useMe } from "../lib/useMe";
 import { api, type DownloadJob } from "../lib/api";
 import { invalidateDownloads } from "../lib/downloads";
 import { inputCls } from "./ui/form";
+import { Button } from "./ui/Button";
 import { DownloadRow, IconBtn, UsageBar } from "./downloadRow";
 import { MetaEditModal, ErrorModal } from "./downloadModals";
 import { AdminSystem } from "./downloadAdmin";
@@ -51,7 +52,13 @@ export default function DownloadCenter() {
   const [editJob, setEditJob] = useState<DownloadJob | null>(null);
   const [errorJob, setErrorJob] = useState<DownloadJob | null>(null);
 
-  const jobsQ = useLiveQuery(qk.downloads(), api.downloads, { intervalMs: 2000 });
+  // Poll fast (2s) while work is actually in flight; back off to 15s once the queue is settled
+  // (only done/error/paused rows left, which don't advance on their own) so an idle Download Center
+  // isn't hitting the API — and adding to the shared box's load — every 2s for nothing.
+  const jobsQ = useLiveQuery(qk.downloads(), api.downloads, {
+    intervalMs: (data) =>
+      data?.some((j) => j.status === "queued" || j.status === "running") ? 2000 : 15000,
+  });
   const sharedQ = useQuery({
     queryKey: qk.downloadsShared(),
     queryFn: api.downloadsShared,
@@ -163,20 +170,24 @@ export default function DownloadCenter() {
                 placeholder={t("downloads.page.addUrl")}
                 className={inputCls}
               />
-              <button
+              <Button
+                variant="ghost"
+                size="sm"
+                iconOnly
+                icon={<ClipboardPaste className="w-4 h-4" />}
+                label={t("downloads.page.paste")}
                 onClick={() => pasteFromClipboard(true)}
-                title={t("downloads.page.paste")}
-                aria-label={t("downloads.page.paste")}
-                className="inline-flex items-center justify-center px-2.5 py-1.5 rounded-lg text-muted hover:text-fg hover:bg-surface transition shrink-0"
-              >
-                <ClipboardPaste className="w-4 h-4" />
-              </button>
-              <button
+                className="shrink-0"
+              />
+              <Button
+                variant="primary"
+                size="sm"
+                icon={<Plus className="w-4 h-4" />}
                 onClick={() => addUrl.trim() && setAddSource(addUrl.trim())}
-                className="inline-flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm font-medium btn-primary transition shrink-0"
+                className="shrink-0"
               >
-                <Plus className="w-4 h-4" /> {t("downloads.page.add")}
-              </button>
+                {t("downloads.page.add")}
+              </Button>
             </div>
             <div className="space-y-2">
               {queue.map((job) => (
diff --git a/frontend/src/components/DownloadDialog.tsx b/frontend/src/components/DownloadDialog.tsx
index 0fc3f72e..1c6d3eb4 100644
--- a/frontend/src/components/DownloadDialog.tsx
+++ b/frontend/src/components/DownloadDialog.tsx
@@ -3,6 +3,7 @@ import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import Modal from "./Modal";
+import { Button } from "./ui/Button";
 import { api } from "../lib/api";
 import { invalidateDownloads } from "../lib/downloads";
 
@@ -89,19 +90,12 @@ export default function DownloadDialog({
       />
 
       <div className="flex justify-end gap-2">
-        <button
-          onClick={onClose}
-          className="px-3 py-1.5 rounded-lg text-sm text-muted hover:text-fg hover:bg-surface transition"
-        >
+        <Button variant="ghost" size="sm" onClick={onClose}>
           {t("downloads.actions.cancel")}
-        </button>
-        <button
-          onClick={submit}
-          disabled={busy || !chosen}
-          className="px-3 py-1.5 rounded-lg text-sm font-medium btn-primary disabled:opacity-50 transition"
-        >
+        </Button>
+        <Button variant="primary" size="sm" onClick={submit} disabled={busy || !chosen}>
           {t("downloads.dialog.submit")}
-        </button>
+        </Button>
       </div>
 
       {manage && (
diff --git a/frontend/src/components/ErrorDialog.tsx b/frontend/src/components/ErrorDialog.tsx
index 6ba217a6..5b8968bb 100644
--- a/frontend/src/components/ErrorDialog.tsx
+++ b/frontend/src/components/ErrorDialog.tsx
@@ -2,6 +2,7 @@ import { useSyncExternalStore } from "react";
 import { useTranslation } from "react-i18next";
 import { dismissError, getError, subscribeError } from "../lib/errorDialog";
 import Modal from "./Modal";
+import { Button } from "./ui/Button";
 
 // Renders the current app error (if any) as a modal the user must acknowledge.
 export default function ErrorDialog() {
@@ -12,12 +13,7 @@ export default function ErrorDialog() {
     <Modal title={err.title} onClose={dismissError} maxWidth="max-w-md">
       <p className="text-sm leading-relaxed text-fg/90">{err.message}</p>
       <div className="flex justify-end mt-5">
-        <button
-          onClick={dismissError}
-          className="px-4 py-2 rounded-lg btn-primary text-sm font-medium transition"
-        >
-          {t("errors.ok")}
-        </button>
+        <Button onClick={dismissError}>{t("errors.ok")}</Button>
       </div>
     </Modal>
   );
diff --git a/frontend/src/components/PersonalViewsModal.tsx b/frontend/src/components/PersonalViewsModal.tsx
index 2e85c361..2a66c37e 100644
--- a/frontend/src/components/PersonalViewsModal.tsx
+++ b/frontend/src/components/PersonalViewsModal.tsx
@@ -18,6 +18,7 @@ import {
 import { CSS } from "@dnd-kit/utilities";
 import { coercePlexFilters, type FeedFilters, type PlexFilters, type SavedView } from "../lib/api";
 import Modal from "./Modal";
+import { Button } from "./ui/Button";
 import { useSavedViews, asFilters } from "./SavedViewsProvider";
 import { useActiveFilters } from "../lib/useActiveFilters";
 import { buildPlexActiveFilters } from "./usePlexActiveFilters";
@@ -202,14 +203,16 @@ export default function PersonalViewsModal() {
               placeholder={t("views.namePlaceholder")}
               className="min-w-0 flex-1 rounded-lg border border-border bg-surface px-2 py-1.5 text-sm focus:border-accent focus:outline-hidden focus:ring-2 focus:ring-accent/30"
             />
-            <button
+            <Button
+              variant="primary"
+              size="sm"
+              iconOnly
+              icon={<BookmarkPlus className="h-4 w-4" />}
+              label={t("views.save")}
               onClick={onCreate}
               disabled={!createName.trim()}
-              title={t("views.save")}
-              className="inline-flex shrink-0 items-center gap-1 rounded-lg btn-primary px-2.5 py-1.5 text-sm font-semibold disabled:opacity-40"
-            >
-              <BookmarkPlus className="h-4 w-4" />
-            </button>
+              className="shrink-0"
+            />
           </div>
         </div>
 
@@ -272,13 +275,14 @@ export default function PersonalViewsModal() {
               )}
             </div>
             <div className="mt-auto flex flex-wrap items-center gap-2 border-t border-border/60 pt-3">
-              <button
+              <Button
+                variant="primary"
+                size="sm"
+                icon={<Play className="h-3.5 w-3.5" />}
                 onClick={() => apply(selected)}
-                className="inline-flex items-center gap-1.5 rounded-lg btn-primary px-3 py-1.5 text-sm font-semibold"
               >
-                <Play className="h-3.5 w-3.5" />
                 {t("views.apply")}
-              </button>
+              </Button>
               <button
                 onClick={async () => {
                   const saved = await saveCurrent(selected.id);
diff --git a/frontend/src/components/Playlists.tsx b/frontend/src/components/Playlists.tsx
index 466d8c8a..977e2926 100644
--- a/frontend/src/components/Playlists.tsx
+++ b/frontend/src/components/Playlists.tsx
@@ -362,7 +362,15 @@ export default function Playlists() {
     setPushing(true);
     try {
       const plan = await api.playlistPushPlan(selectedId);
-      if (plan.action === "update" && !plan.to_insert && !plan.to_delete && !plan.to_reorder) {
+      const nothingToDo =
+        plan.action === "update" &&
+        !plan.add_yt &&
+        !plan.add_local &&
+        !plan.remove_yt &&
+        !plan.remove_local &&
+        !plan.to_reorder &&
+        !plan.rename;
+      if (nothingToDo) {
         notify({ level: "info", message: t("playlists.pushUpToDate", { name: plName(detail) }) });
         return;
       }
@@ -376,21 +384,37 @@ export default function Playlists() {
         });
         return;
       }
-      let message =
-        plan.action === "create"
-          ? t("playlists.pushPlanCreate", {
-              count: plan.to_insert,
-              units: plan.units_estimate,
-              left: plan.remaining_today,
-            })
-          : t("playlists.pushPlanUpdate", {
-              insert: plan.to_insert,
-              del: plan.to_delete,
-              reorder: plan.to_reorder,
-              units: plan.units_estimate,
-              left: plan.remaining_today,
-            });
-      if (plan.yt_extra > 0) message += " " + t("playlists.pushDiverged", { count: plan.yt_extra });
+      let message: string;
+      if (plan.action === "create") {
+        message = t("playlists.pushPlanCreate", {
+          count: plan.add_yt,
+          units: plan.units_estimate,
+          left: plan.remaining_today,
+        });
+      } else {
+        // One sentence per side that actually changes; the untouched side gets a short "stays
+        // unchanged" reassurance instead of a "+0 / −0" the reader has to decode.
+        const here: string[] = [];
+        if (plan.add_local) here.push(t("playlists.mergeAddLocal", { count: plan.add_local }));
+        if (plan.remove_local)
+          here.push(t("playlists.mergeRemoveLocal", { count: plan.remove_local }));
+        const yt: string[] = [];
+        if (plan.add_yt) yt.push(t("playlists.mergeAddYt", { count: plan.add_yt }));
+        if (plan.remove_yt) yt.push(t("playlists.mergeRemoveYt", { count: plan.remove_yt }));
+        if (plan.to_reorder) yt.push(t("playlists.mergeReorder", { count: plan.to_reorder }));
+        if (plan.rename) yt.push(t("playlists.mergeRename"));
+        const sentences = [...here, ...yt];
+        if (here.length && !yt.length) sentences.push(t("playlists.mergeYtUnchanged"));
+        if (yt.length && !here.length) sentences.push(t("playlists.mergeLocalUnchanged"));
+        const quota =
+          plan.units_estimate > 0
+            ? t("playlists.mergeQuota", {
+                units: plan.units_estimate,
+                left: plan.remaining_today,
+              })
+            : t("playlists.mergeQuotaFree");
+        message = sentences.join(" ") + quota;
+      }
       const ok = await confirm({
         title: t("playlists.pushTitle"),
         message,
diff --git a/frontend/src/components/ProfileEditor.tsx b/frontend/src/components/ProfileEditor.tsx
index 8822ca19..b043d5a2 100644
--- a/frontend/src/components/ProfileEditor.tsx
+++ b/frontend/src/components/ProfileEditor.tsx
@@ -7,6 +7,7 @@ import Modal from "./Modal";
 import { useConfirm } from "./ConfirmProvider";
 import { api, type DownloadProfile, type DownloadSpec } from "../lib/api";
 import { inputCls } from "./ui/form";
+import { Button } from "./ui/Button";
 
 const HEIGHTS = [null, 2160, 1440, 1080, 720, 480, 360];
 
@@ -145,12 +146,9 @@ export default function ProfileEditor({ onClose }: { onClose: () => void }) {
       </div>
 
       {!formOpen ? (
-        <button
-          onClick={startNew}
-          className="inline-flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm font-medium btn-primary transition"
-        >
-          <Plus className="w-4 h-4" /> {t("downloads.profiles.new")}
-        </button>
+        <Button variant="primary" size="sm" icon={<Plus className="w-4 h-4" />} onClick={startNew}>
+          {t("downloads.profiles.new")}
+        </Button>
       ) : (
         <div className="rounded-xl border border-border p-4 space-y-3">
           <div>
@@ -263,19 +261,17 @@ export default function ProfileEditor({ onClose }: { onClose: () => void }) {
             />
           </div>
           <div className="flex justify-end gap-2 pt-1">
-            <button
-              onClick={() => setFormOpen(false)}
-              className="px-3 py-1.5 rounded-lg text-sm text-muted hover:text-fg hover:bg-surface transition"
-            >
+            <Button variant="ghost" size="sm" onClick={() => setFormOpen(false)}>
               {t("downloads.actions.cancel")}
-            </button>
-            <button
+            </Button>
+            <Button
+              variant="primary"
+              size="sm"
               onClick={() => save.mutate()}
               disabled={!name.trim() || save.isPending}
-              className="px-3 py-1.5 rounded-lg text-sm font-medium btn-primary disabled:opacity-50 transition"
             >
               {editing ? t("downloads.profiles.save") : t("downloads.profiles.create")}
-            </button>
+            </Button>
           </div>
         </div>
       )}
diff --git a/frontend/src/components/SettingsPanel.downloadAuth.dom.test.tsx b/frontend/src/components/SettingsPanel.downloadAuth.dom.test.tsx
new file mode 100644
index 00000000..2d87c068
--- /dev/null
+++ b/frontend/src/components/SettingsPanel.downloadAuth.dom.test.tsx
@@ -0,0 +1,113 @@
+// @vitest-environment jsdom
+//
+// S6 — the per-user download-cookies section is WRITE-ONLY: the stored blob is never fetched or
+// shown back, so opening "Set"/"Update" always reveals an EMPTY field, and saving posts exactly the
+// pasted value to the matching host. This drives the real DownloadAuth against a mocked api so we can
+// assert the field never carries a prior value and the right host is written.
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { render, screen, cleanup, fireEvent, waitFor } from "@testing-library/react";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+
+const h = vi.hoisted(() => ({
+  downloadCredentials: vi.fn(),
+  setDownloadCredential: vi.fn(),
+  deleteDownloadCredential: vi.fn(),
+}));
+
+vi.mock("react-i18next", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("react-i18next")>();
+  return {
+    ...actual,
+    useTranslation: () => ({ t: (key: string) => key, i18n: { language: "en" } }),
+  };
+});
+vi.mock("../lib/api", () => ({ api: h }));
+// Partial mock: SettingsPanel's module graph (PrefsProvider) also pulls getNotifSettings from here.
+vi.mock("../lib/notifications", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("../lib/notifications")>();
+  return { ...actual, notify: vi.fn() };
+});
+vi.mock("./ConfirmProvider", () => ({ useConfirm: () => vi.fn(async () => true) }));
+
+import { DownloadAuth } from "./SettingsPanel";
+
+const HOSTS = [
+  { host_key: "facebook", label: "Facebook", configured: false, updated_at: null },
+  {
+    host_key: "instagram",
+    label: "Instagram",
+    configured: true,
+    updated_at: "2026-08-01T00:00:00Z",
+  },
+];
+
+function renderPanel() {
+  const qc = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+  return render(
+    <QueryClientProvider client={qc}>
+      <DownloadAuth />
+    </QueryClientProvider>,
+  );
+}
+
+describe("DownloadAuth (write-only cookies)", () => {
+  beforeEach(() => {
+    h.downloadCredentials.mockResolvedValue({ hosts: HOSTS });
+    h.setDownloadCredential.mockResolvedValue({ ok: true, hosts: HOSTS });
+    h.deleteDownloadCredential.mockResolvedValue({ ok: true, hosts: HOSTS });
+  });
+  afterEach(cleanup);
+
+  it("lists every supported host with its configured state", async () => {
+    renderPanel();
+    await screen.findByTestId("cookie-host-facebook");
+    expect(screen.getByTestId("cookie-status-facebook").textContent).toContain(
+      "settings.downloadAuth.notSet",
+    );
+    expect(screen.getByTestId("cookie-status-instagram").textContent).toContain(
+      "settings.downloadAuth.configured",
+    );
+  });
+
+  it("opens an EMPTY field on Set and posts the pasted value to that host", async () => {
+    renderPanel();
+    await screen.findByTestId("cookie-host-facebook");
+    // The Set button is the outline action on the facebook row.
+    const setBtn = screen.getByText("settings.downloadAuth.set");
+    fireEvent.click(setBtn);
+    const field = (await screen.findByTestId("cookie-input-facebook")) as HTMLTextAreaElement;
+    expect(field.value).toBe(""); // write-only: never prefilled
+
+    fireEvent.change(field, {
+      target: { value: "# Netscape\n.facebook.com\tTRUE\t/\tT\t0\tc\t1\n" },
+    });
+    fireEvent.click(screen.getByText("settings.downloadAuth.save"));
+    await waitFor(() =>
+      expect(h.setDownloadCredential).toHaveBeenCalledWith(
+        "facebook",
+        "# Netscape\n.facebook.com\tTRUE\t/\tT\t0\tc\t1\n",
+      ),
+    );
+  });
+
+  it("re-opening Update on a configured host still shows an empty field", async () => {
+    renderPanel();
+    await screen.findByTestId("cookie-host-instagram");
+    fireEvent.click(screen.getByText("settings.downloadAuth.update"));
+    const field = (await screen.findByTestId("cookie-input-instagram")) as HTMLTextAreaElement;
+    expect(field.value).toBe(""); // the stored blob is never fetched into the field
+  });
+
+  it("renders an honest error state with retry when the fetch fails, not an empty section", async () => {
+    // Honest-states: a failed load must NOT render as a bare empty section (no host rows).
+    h.downloadCredentials.mockRejectedValueOnce(new Error("boom"));
+    h.downloadCredentials.mockResolvedValue({ hosts: HOSTS });
+    renderPanel();
+    // The error message shows and no host rows render.
+    await screen.findByText("settings.downloadAuth.loadError");
+    expect(screen.queryByTestId("cookie-host-facebook")).toBeNull();
+    // Retry refetches → the list appears.
+    fireEvent.click(screen.getByText("common.retry"));
+    await screen.findByTestId("cookie-host-facebook");
+  });
+});
diff --git a/frontend/src/components/SettingsPanel.tsx b/frontend/src/components/SettingsPanel.tsx
index 4b6f233b..c11d7cf4 100644
--- a/frontend/src/components/SettingsPanel.tsx
+++ b/frontend/src/components/SettingsPanel.tsx
@@ -11,8 +11,10 @@ import Avatar from "./Avatar";
 import { notify, type NotifSettings } from "../lib/notifications";
 import Tooltip from "./Tooltip";
 import { inputCls, Section, SettingRow, Switch } from "./ui/form";
+import { Button } from "./ui/Button";
 import { type SaveState } from "./ui/DraftSaveBar";
 import { useConfirm } from "./ConfirmProvider";
+import { StateMessage } from "./QueryState";
 import { useWizard } from "./WizardProvider";
 import { usePrefs } from "./PrefsProvider";
 import Tabs from "./Tabs";
@@ -472,13 +474,9 @@ function SignInMethods({ me }: { me: Me }) {
               className={inputCls}
             />
             {err && <p className="text-xs text-danger">{err}</p>}
-            <button
-              type="submit"
-              disabled={busy || !next}
-              className="px-3 py-1.5 rounded-lg text-sm btn-primary font-medium disabled:opacity-40 transition"
-            >
+            <Button type="submit" variant="primary" size="sm" disabled={busy || !next}>
               {busy ? t("settings.account.password.saving") : t("settings.account.password.save")}
-            </button>
+            </Button>
           </form>
         )}
       </div>
@@ -586,6 +584,161 @@ function PlexWatchSync() {
   );
 }
 
+// Per-user download authentication (S6): store your own login cookies for an auth-walled host
+// (e.g. a private Facebook group you belong to) so downloads from it use YOUR login. Write-only —
+// the stored blob is never shown back; each row shows only the host + whether it's set + when.
+// A cookie-authed download is kept private to your account (never cache-shared with another user).
+// Exported for a focused DOM test (write-only behaviour); rendered inside Account below.
+export function DownloadAuth() {
+  const { t, i18n } = useTranslation();
+  const qc = useQueryClient();
+  const confirm = useConfirm();
+  const q = useQuery({
+    queryKey: qk.downloadCredentials(),
+    queryFn: () => api.downloadCredentials(),
+  });
+  const [openHost, setOpenHost] = useState<string | null>(null);
+  const [draft, setDraft] = useState("");
+
+  const save = useMutation({
+    mutationFn: ({ host, cookies }: { host: string; cookies: string }) =>
+      api.setDownloadCredential(host, cookies),
+    onSuccess: (res) => {
+      qc.setQueryData(qk.downloadCredentials(), res);
+      setOpenHost(null);
+      setDraft("");
+      notify({ level: "success", message: t("settings.downloadAuth.saved") });
+    },
+    // Validation errors (empty / not-a-cookies.txt) surface via the global error modal, which
+    // localizes the structured { code: "cookies" } detail (see lib/api core localizeDetail).
+  });
+  const remove = useMutation({
+    mutationFn: (host: string) => api.deleteDownloadCredential(host),
+    onSuccess: (res) => {
+      qc.setQueryData(qk.downloadCredentials(), res);
+      notify({ level: "success", message: t("settings.downloadAuth.removed") });
+    },
+  });
+
+  const hosts = q.data?.hosts ?? [];
+  const onRemove = async (host: string) => {
+    const ok = await confirm({
+      title: t("settings.downloadAuth.removeConfirmTitle"),
+      message: t("settings.downloadAuth.removeConfirmBody"),
+      confirmLabel: t("settings.downloadAuth.removeConfirmAction"),
+      danger: true,
+    });
+    if (ok) remove.mutate(host);
+  };
+
+  return (
+    <Section title={t("settings.downloadAuth.title")}>
+      <p className="text-xs text-muted leading-relaxed mb-2">{t("settings.downloadAuth.intro")}</p>
+      {/* The host list is server-provided (always non-empty on success), so an empty `hosts` means
+          the fetch failed or is still loading — render an honest state, never a bare empty section
+          that reads as "nothing to configure here". */}
+      {q.isError && hosts.length === 0 ? (
+        <StateMessage tone="error" onRetry={() => q.refetch()}>
+          {t("settings.downloadAuth.loadError")}
+        </StateMessage>
+      ) : q.isLoading ? (
+        <StateMessage>{t("common.loading")}</StateMessage>
+      ) : (
+        <div className="divide-y divide-border">
+          {hosts.map((h) => {
+            const when = h.updated_at
+              ? new Date(h.updated_at).toLocaleDateString(i18n.language)
+              : "";
+            const open = openHost === h.host_key;
+            return (
+              <div key={h.host_key} className="py-2" data-testid={`cookie-host-${h.host_key}`}>
+                <div className="flex items-start justify-between gap-3">
+                  <div className="min-w-0">
+                    <div className="text-sm font-medium">{h.label}</div>
+                    <p
+                      className="text-xs text-muted mt-0.5"
+                      data-testid={`cookie-status-${h.host_key}`}
+                    >
+                      {h.configured
+                        ? t("settings.downloadAuth.configured", { when })
+                        : t("settings.downloadAuth.notSet")}
+                    </p>
+                  </div>
+                  <div className="flex items-center gap-2 shrink-0">
+                    {h.configured && (
+                      <Button
+                        variant="danger-outline"
+                        size="sm"
+                        onClick={() => onRemove(h.host_key)}
+                        disabled={remove.isPending}
+                      >
+                        {t("settings.downloadAuth.remove")}
+                      </Button>
+                    )}
+                    <Button
+                      variant="outline"
+                      size="sm"
+                      onClick={() => {
+                        setOpenHost(open ? null : h.host_key);
+                        setDraft("");
+                      }}
+                    >
+                      {h.configured
+                        ? t("settings.downloadAuth.update")
+                        : t("settings.downloadAuth.set")}
+                    </Button>
+                  </div>
+                </div>
+                {open && (
+                  <div className="mt-2 space-y-2">
+                    {/* Write-only: the field is ALWAYS empty on open — the stored blob is never
+                      fetched or prefilled. */}
+                    <textarea
+                      value={draft}
+                      onChange={(e) => setDraft(e.target.value)}
+                      rows={5}
+                      placeholder={t("settings.downloadAuth.placeholder")}
+                      className={`${inputCls} font-mono text-xs resize-y`}
+                      data-testid={`cookie-input-${h.host_key}`}
+                      spellCheck={false}
+                      autoComplete="off"
+                    />
+                    <p className="text-2xs text-muted leading-relaxed">
+                      {t("settings.downloadAuth.help")}
+                    </p>
+                    <div className="flex items-center gap-2">
+                      <Button
+                        variant="primary"
+                        size="sm"
+                        disabled={!draft.trim() || save.isPending}
+                        onClick={() => save.mutate({ host: h.host_key, cookies: draft })}
+                      >
+                        {save.isPending
+                          ? t("settings.downloadAuth.saving")
+                          : t("settings.downloadAuth.save")}
+                      </Button>
+                      <Button
+                        variant="ghost"
+                        size="sm"
+                        onClick={() => {
+                          setOpenHost(null);
+                          setDraft("");
+                        }}
+                      >
+                        {t("settings.downloadAuth.cancel")}
+                      </Button>
+                    </div>
+                  </div>
+                )}
+              </div>
+            );
+          })}
+        </div>
+      )}
+    </Section>
+  );
+}
+
 function Account({ me, onOpenWizard }: { me: Me; onOpenWizard: () => void }) {
   const { t } = useTranslation();
   const confirm = useConfirm();
@@ -628,6 +781,8 @@ function Account({ me, onOpenWizard }: { me: Me; onOpenWizard: () => void }) {
 
       {me.role === "admin" && me.plex_enabled && <PlexWatchSync />}
 
+      {!me.is_demo && <DownloadAuth />}
+
       {me.is_demo ? (
         <Section title={t("settings.account.youtubeAccess")}>
           <p className="text-xs text-muted leading-relaxed">{t("settings.account.demoNotice")}</p>
diff --git a/frontend/src/components/ShareDialog.tsx b/frontend/src/components/ShareDialog.tsx
index b649feda..41e58e19 100644
--- a/frontend/src/components/ShareDialog.tsx
+++ b/frontend/src/components/ShareDialog.tsx
@@ -9,7 +9,8 @@ import { useConfirm } from "./ConfirmProvider";
 import { notify } from "../lib/notifications";
 import { copyText } from "../lib/clipboard";
 import { api, type DownloadJob, type ShareLink } from "../lib/api";
-import { inputCls, btnCls } from "./ui/form";
+import { inputCls } from "./ui/form";
+import { Button } from "./ui/Button";
 const EXPIRY_DAYS = [null, 1, 7, 30] as const;
 
 // --- share with a registered user (ACL) -----------------------------------------------------
@@ -343,31 +344,29 @@ function LinkShare({ job }: { job: DownloadJob }) {
             autoComplete="new-password"
           />
           <div className="flex justify-end gap-2">
-            <button
-              onClick={() => setCreating(false)}
-              className={clsx(btnCls, "text-muted hover:text-fg hover:bg-surface")}
-            >
+            <Button variant="ghost" size="sm" onClick={() => setCreating(false)}>
               {t("common.cancel")}
-            </button>
-            <button
+            </Button>
+            <Button
+              variant="primary"
+              size="sm"
               onClick={() => create.mutate()}
               disabled={create.isPending}
-              className={clsx(btnCls, "btn-primary")}
             >
               {t("downloads.share.createLink")}
-            </button>
+            </Button>
           </div>
         </div>
       ) : (
-        <button
+        <Button
+          variant="outline"
+          size="sm"
           onClick={() => setCreating(true)}
-          className={clsx(
-            btnCls,
-            "mt-2 border border-border text-muted hover:text-fg hover:border-muted inline-flex items-center gap-1.5",
-          )}
+          icon={<Link2 className="w-4 h-4" />}
+          className="mt-2"
         >
-          <Link2 className="w-4 h-4" /> {t("downloads.share.newLink")}
-        </button>
+          {t("downloads.share.newLink")}
+        </Button>
       )}
     </div>
   );
diff --git a/frontend/src/components/SortControl.tsx b/frontend/src/components/SortControl.tsx
index 817b0dc7..60117335 100644
--- a/frontend/src/components/SortControl.tsx
+++ b/frontend/src/components/SortControl.tsx
@@ -55,7 +55,7 @@ export default function SortControl({
   const summary = activeOpt?.label ?? defaultLabel ?? options[0]?.label ?? "";
 
   const segCls = (on: boolean) =>
-    `flex flex-1 items-center justify-center gap-1 rounded-md border px-2 py-1 text-2xs transition ${
+    `flex flex-1 items-center justify-center gap-1 whitespace-nowrap rounded-md border px-2 py-1 text-2xs transition ${
       on
         ? "border-accent bg-accent/15 text-accent"
         : "border-border text-muted hover:border-accent/50 hover:text-fg"
@@ -92,7 +92,7 @@ export default function SortControl({
       <PopoverSurface
         popover={pop}
         ariaLabel={label}
-        className="glass-menu min-w-52 rounded-xl p-1.5 shadow-lg animate-[popIn_0.16s_ease]"
+        className="glass-menu min-w-56 rounded-xl p-1.5 shadow-lg animate-[popIn_0.16s_ease]"
       >
         {/* One exclusive choice of sort key (radios), scoped by a group so AT announces which is
             checked — the surface is a menu (see the mode above), never a listbox with orphaned options. */}
diff --git a/frontend/src/components/VideoEditor.tsx b/frontend/src/components/VideoEditor.tsx
index b23de353..bab983f6 100644
--- a/frontend/src/components/VideoEditor.tsx
+++ b/frontend/src/components/VideoEditor.tsx
@@ -2,22 +2,13 @@ import { useCallback, useEffect, useMemo, useRef, useState } from "react";
 import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
-import {
-  Crop,
-  Eye,
-  EyeOff,
-  Pause,
-  Play,
-  Scissors,
-  SplitSquareHorizontal,
-  Loader2,
-  X,
-} from "lucide-react";
+import { Crop, Eye, EyeOff, Pause, Play, Scissors, SplitSquareHorizontal, X } from "lucide-react";
 import clsx from "clsx";
 import Modal from "./Modal";
 import { notify } from "../lib/notifications";
 import { api, type DownloadJob, type EditSpec } from "../lib/api";
 import { inputCls, btnCls } from "./ui/form";
+import { Button } from "./ui/Button";
 
 // mm:ss.d timecode <-> seconds.
 function tc(s: number): string {
@@ -621,28 +612,23 @@ export default function VideoEditor({ job, onClose }: { job: DownloadJob; onClos
             {reencode ? t("editor.willReencode") : t("editor.willCopy")}
           </span>
           <div className="flex gap-2">
-            <button
-              onClick={onClose}
-              className={clsx(btnCls, "text-muted hover:text-fg hover:bg-surface")}
-            >
+            <Button variant="ghost" size="sm" onClick={onClose}>
               {t("common.cancel")}
-            </button>
-            <button
+            </Button>
+            <Button
+              variant="primary"
+              size="sm"
               onClick={() => create.mutate()}
-              disabled={!valid || create.isPending}
-              className={clsx(btnCls, "btn-primary inline-flex items-center gap-1.5")}
+              disabled={!valid}
+              loading={create.isPending}
+              icon={<Scissors className="w-4 h-4" />}
             >
-              {create.isPending ? (
-                <Loader2 className="w-4 h-4 animate-spin" />
-              ) : (
-                <Scissors className="w-4 h-4" />
-              )}
               {willJoin
                 ? t("editor.createJoin")
                 : kept.length > 1
                   ? t("editor.createN", { count: kept.length })
                   : t("editor.create")}
-            </button>
+            </Button>
           </div>
         </div>
       </div>
diff --git a/frontend/src/components/WatchPage.dom.test.tsx b/frontend/src/components/WatchPage.dom.test.tsx
new file mode 100644
index 00000000..74e0b1c1
--- /dev/null
+++ b/frontend/src/components/WatchPage.dom.test.tsx
@@ -0,0 +1,39 @@
+// @vitest-environment jsdom
+//
+// S3 item 5a: the public watch page must render a native <audio> element for an audio-only asset
+// (mp3) and a <video> for everything else. WebKit (all iOS browsers) refuses to play an audio-only
+// mp3 inside a <video>, which is exactly why the branch exists — this locks it so a refactor can't
+// silently drop the audio player and reintroduce the iPad-broken behavior.
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { render, waitFor, cleanup } from "@testing-library/react";
+import WatchPage from "./WatchPage";
+
+function mockMeta(meta: Record<string, unknown>) {
+  vi.stubGlobal(
+    "fetch",
+    vi.fn(async () => ({ ok: true, json: async () => meta })),
+  );
+}
+
+afterEach(() => {
+  cleanup();
+  vi.unstubAllGlobals();
+});
+
+describe("WatchPage media element (S3 · 5a)", () => {
+  it("renders a native <audio> for an audio-only asset", async () => {
+    window.history.replaceState({}, "", "/watch/tok-audio");
+    mockMeta({ needs_password: false, audio: true, file_url: "/f.mp3", title: "A song" });
+    const { container } = render(<WatchPage />);
+    await waitFor(() => expect(container.querySelector("audio")).not.toBeNull());
+    expect(container.querySelector("video")).toBeNull();
+  });
+
+  it("renders a <video> for a normal video asset", async () => {
+    window.history.replaceState({}, "", "/watch/tok-video");
+    mockMeta({ needs_password: false, audio: false, file_url: "/f.mp4", title: "A clip" });
+    const { container } = render(<WatchPage />);
+    await waitFor(() => expect(container.querySelector("video")).not.toBeNull());
+    expect(container.querySelector("audio")).toBeNull();
+  });
+});
diff --git a/frontend/src/components/WatchPage.tsx b/frontend/src/components/WatchPage.tsx
index 41d6917b..6ff7b9c3 100644
--- a/frontend/src/components/WatchPage.tsx
+++ b/frontend/src/components/WatchPage.tsx
@@ -1,5 +1,6 @@
 import { useEffect, useRef, useState } from "react";
 import { Download, Link2, Lock, PlayCircle } from "lucide-react";
+import { Button } from "./ui/Button";
 
 // Standalone, login-free video page for a shared /watch/<token> link. Rendered OUTSIDE the app
 // tree (no auth, no providers) — like the /privacy and /terms pages. Uses plain fetch and a tiny
@@ -11,10 +12,11 @@ import { Download, Link2, Lock, PlayCircle } from "lucide-react";
 // playing in the background on Brave iOS. We deliberately do NOT route the audio through the Web
 // Audio API: `createMediaElementSource` pulls the element out of the normal media pipeline that the
 // background feature (and iOS) hooks into, and iOS then suspends the AudioContext on background —
-// which STOPPED playback instead of keeping it alive (measured on iPad Brave). A plain <video> +
-// MediaSession leaves the browser's native background handling in charge.
+// which STOPPED playback instead of keeping it alive (measured on iPad Brave). A plain <video>/<audio>
+// + MediaSession leaves the browser's native background handling in charge. Typed on HTMLMediaElement
+// so it drives either the video or the audio-only player.
 function useMediaSession(
-  videoRef: React.RefObject<HTMLVideoElement | null>,
+  videoRef: React.RefObject<HTMLMediaElement | null>,
   meta: Meta | null,
   ready: boolean,
 ) {
@@ -45,6 +47,7 @@ type Meta = {
   source_url?: string | null;
   extra_links?: string[];
   duration_s?: number | null;
+  audio?: boolean;
   allow_download?: boolean;
   file_url?: string;
   poster_url?: string | null;
@@ -112,8 +115,13 @@ export default function WatchPage() {
   const [password, setPassword] = useState("");
   const [error, setError] = useState<string | null>(null);
   const [unlocking, setUnlocking] = useState(false);
-  const videoRef = useRef<HTMLVideoElement>(null);
-  useMediaSession(videoRef, meta, status === "ready");
+  // One ref for whichever element renders (video or audio-only) — a callback ref so it can bind to
+  // both without a per-element typed ref.
+  const mediaRef = useRef<HTMLMediaElement | null>(null);
+  const setMedia = (el: HTMLMediaElement | null) => {
+    mediaRef.current = el;
+  };
+  useMediaSession(mediaRef, meta, status === "ready");
 
   useEffect(() => {
     if (!token) {
@@ -193,31 +201,43 @@ export default function WatchPage() {
                 className="w-full rounded-lg bg-surface border border-border px-3 py-2 text-sm outline-hidden focus:ring-2 focus:ring-accent/40"
               />
               {error && <div className="text-sm text-danger mt-2">{error}</div>}
-              <button
+              <Button
+                variant="primary"
+                size="md"
+                fullWidth
                 onClick={unlock}
                 disabled={!password || unlocking}
-                className="mt-4 w-full rounded-lg btn-primary disabled:opacity-50 py-2 text-sm font-medium transition"
+                className="mt-4"
               >
                 {T.watch}
-              </button>
+              </Button>
             </div>
           </div>
         )}
 
         {status === "ready" && meta && (
           <div>
-            {/* Shrink-wrap the player to the video so a vertical/short clip renders as a narrow
-                player instead of a wide box with black bars; a landscape clip fills the width. */}
-            <div className="rounded-xl overflow-hidden bg-black border border-border w-fit max-w-full mx-auto">
-              <video
-                ref={videoRef}
-                controls
-                playsInline
-                src={meta.file_url}
-                poster={meta.poster_url || undefined}
-                className="max-h-[80vh] max-w-full mx-auto block"
-              />
-            </div>
+            {meta.audio ? (
+              /* Audio-only asset (mp3): WebKit won't play it in a <video>, so render a native
+                 <audio>. No poster exists for these, so present it as a deliberate padded bar on
+                 the surface (not the black video box) so it looks intentional without artwork. */
+              <div className="rounded-xl border border-border bg-surface p-4">
+                <audio ref={setMedia} controls src={meta.file_url} className="w-full block" />
+              </div>
+            ) : (
+              /* Shrink-wrap the player to the video so a vertical/short clip renders as a narrow
+                 player instead of a wide box with black bars; a landscape clip fills the width. */
+              <div className="rounded-xl overflow-hidden bg-black border border-border w-fit max-w-full mx-auto">
+                <video
+                  ref={setMedia}
+                  controls
+                  playsInline
+                  src={meta.file_url}
+                  poster={meta.poster_url || undefined}
+                  className="max-h-[80vh] max-w-full mx-auto block"
+                />
+              </div>
+            )}
             <div className="mt-3 flex items-start gap-3">
               <div className="min-w-0 flex-1">
                 <h1 className="text-lg font-semibold leading-snug">{meta.title || "Video"}</h1>
diff --git a/frontend/src/components/downloadAdmin.tsx b/frontend/src/components/downloadAdmin.tsx
index c0f4a965..c3f71898 100644
--- a/frontend/src/components/downloadAdmin.tsx
+++ b/frontend/src/components/downloadAdmin.tsx
@@ -8,6 +8,7 @@ import { useLiveQuery } from "../lib/useLiveQuery";
 import { api } from "../lib/api";
 import { formatBytes } from "../lib/format";
 import { inputCls } from "./ui/form";
+import { Button } from "./ui/Button";
 import { StatusPill, jobTitle } from "./downloadRow";
 
 // The admin "System" tab — storage overview, per-user footprint, quota editor, and the global job
@@ -110,19 +111,17 @@ function QuotaModal({
           {t("downloads.admin.unlimited")}
         </label>
         <div className="flex justify-between pt-1">
-          <button
-            onClick={() => reset.mutate()}
-            className="px-3 py-1.5 rounded-lg text-sm text-muted hover:text-fg hover:bg-surface transition"
-          >
+          <Button variant="ghost" size="sm" onClick={() => reset.mutate()}>
             {t("downloads.admin.reset")}
-          </button>
-          <button
+          </Button>
+          <Button
+            variant="primary"
+            size="sm"
             onClick={() => save.mutate()}
             disabled={save.isPending}
-            className="px-3 py-1.5 rounded-lg text-sm font-medium btn-primary disabled:opacity-50 transition"
           >
             {t("downloads.admin.save")}
-          </button>
+          </Button>
         </div>
       </div>
     </Modal>
diff --git a/frontend/src/components/downloadModals.tsx b/frontend/src/components/downloadModals.tsx
index 785836e6..154d58da 100644
--- a/frontend/src/components/downloadModals.tsx
+++ b/frontend/src/components/downloadModals.tsx
@@ -6,6 +6,7 @@ import { Plus, Trash2 } from "lucide-react";
 import Modal from "./Modal";
 import { api, type DownloadJob } from "../lib/api";
 import { inputCls } from "./ui/form";
+import { Button } from "./ui/Button";
 import { IconBtn } from "./downloadRow";
 
 // Per-item user modals split out of DownloadCenter (self-contained, prop-driven): rename/metadata
@@ -101,22 +102,26 @@ export function MetaEditModal({ job, onClose }: { job: DownloadJob; onClose: ()
         </div>
         <p className="text-xs text-muted">{t("downloads.edit.hint")}</p>
         <div className="flex justify-end">
-          <button
+          <Button
+            variant="primary"
+            size="sm"
             onClick={() => save.mutate()}
             disabled={!name.trim() || save.isPending}
-            className="px-3 py-1.5 rounded-lg text-sm font-medium btn-primary disabled:opacity-50 transition"
           >
             {t("downloads.edit.save")}
-          </button>
+          </Button>
         </div>
       </div>
     </Modal>
   );
 }
 
-// Transient failures worth an in-modal Retry button; the rest are permanent (private/geo/DRM/…),
-// where a retry can't help — the queue row still keeps its own retry control regardless.
-const RETRYABLE_ERRORS = ["postprocess", "network", "unknown"];
+// Transient failures worth an in-modal Retry button; the rest are permanent (private/geo/DRM/
+// sign-in/unsupported-host/…), where a retry can't help — the queue row still keeps its own retry
+// control regardless. `unknown` is deliberately NOT here: an unclassified failure is most often an
+// auth wall we couldn't parse (a login-walled Facebook post reads as a bare "no format"), so a
+// prominent Retry would mislead; force-retrying from the row stays available for genuine transients.
+const RETRYABLE_ERRORS = ["postprocess", "network"];
 
 // A readable "why did this fail" window for a failed job: a category headline + plain explanation,
 // the raw yt-dlp/ffmpeg text behind a disclosure, and a Retry when the failure may be transient.
@@ -155,22 +160,20 @@ export function ErrorModal({
         )}
         <div className="flex justify-end gap-2 pt-1">
           {retryable && (
-            <button
+            <Button
+              variant="primary"
+              size="sm"
               onClick={() => {
                 onRetry();
                 onClose();
               }}
-              className="px-3 py-1.5 rounded-lg text-sm font-medium btn-primary transition"
             >
               {t("downloads.actions.retry")}
-            </button>
+            </Button>
           )}
-          <button
-            onClick={onClose}
-            className="px-3 py-1.5 rounded-lg text-sm text-muted hover:text-fg hover:bg-surface transition"
-          >
+          <Button variant="ghost" size="sm" onClick={onClose}>
             {t("common.close")}
-          </button>
+          </Button>
         </div>
       </div>
     </Modal>
diff --git a/frontend/src/components/downloadRow.tsx b/frontend/src/components/downloadRow.tsx
index c9227d3e..53f00711 100644
--- a/frontend/src/components/downloadRow.tsx
+++ b/frontend/src/components/downloadRow.tsx
@@ -278,8 +278,12 @@ export function UsageBar() {
   const { t } = useTranslation();
   // Poll live (like the library list) so the footprint updates the moment the worker finishes an
   // edit/download — enqueue only creates a queued job (0 bytes), the size lands asynchronously, so
-  // a one-shot fetch at enqueue time would show a stale 0 B until a manual reload.
-  const usage = useLiveQuery(qk.downloadUsage(), api.downloadUsage, { intervalMs: 2000 });
+  // a one-shot fetch at enqueue time would show a stale 0 B until a manual reload. The footprint
+  // only moves while a job is in flight, so poll fast (2s) then and idle-back-off to 15s otherwise
+  // (running_jobs counts queued+running — it flips back to 2s on the first poll after a new enqueue).
+  const usage = useLiveQuery(qk.downloadUsage(), api.downloadUsage, {
+    intervalMs: (u) => (u?.running_jobs ? 2000 : 15000),
+  });
   const u = usage.data;
   if (!u) return null;
   const pct =
diff --git a/frontend/src/components/ui/Button.dom.test.tsx b/frontend/src/components/ui/Button.dom.test.tsx
new file mode 100644
index 00000000..6db53608
--- /dev/null
+++ b/frontend/src/components/ui/Button.dom.test.tsx
@@ -0,0 +1,78 @@
+// @vitest-environment jsdom
+//
+// S5: the shared <Button> primitive. These lock the load-bearing contract the 120+ migrated call
+// sites rely on: variant/size paint, icon-only accessibility (name must survive when the text is
+// dropped), the loading state (disabled + spinner + aria-busy), and the type="button" default that
+// stops an action button from submitting a surrounding form.
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { render, screen, cleanup, fireEvent } from "@testing-library/react";
+import { Button } from "./Button";
+
+afterEach(cleanup);
+
+describe("Button primitive (S5)", () => {
+  it("defaults to type=button so it never submits a form", () => {
+    render(<Button>Go</Button>);
+    expect(screen.getByRole("button", { name: "Go" }).getAttribute("type")).toBe("button");
+  });
+
+  it("applies the variant paint recipe", () => {
+    render(
+      <Button variant="danger" size="sm">
+        Delete
+      </Button>,
+    );
+    const btn = screen.getByRole("button", { name: "Delete" });
+    expect(btn.className).toContain("btn-danger");
+    expect(btn.className).toContain("py-1.5");
+  });
+
+  it("exposes an accessible name for an icon-only button while dropping the text", () => {
+    render(
+      <Button iconOnly icon={<svg data-testid="ic" />} label="Block channel">
+        this text is dropped
+      </Button>,
+    );
+    const btn = screen.getByRole("button", { name: "Block channel" });
+    expect(btn.getAttribute("title")).toBe("Block channel");
+    expect(btn.textContent).not.toContain("this text is dropped");
+    expect(btn.querySelector("[data-testid=ic]")).not.toBeNull();
+  });
+
+  it("loading disables the button, marks aria-busy, and blocks clicks", () => {
+    const onClick = vi.fn();
+    render(
+      <Button loading icon={<svg data-testid="ic" />} onClick={onClick}>
+        Save
+      </Button>,
+    );
+    const btn = screen.getByRole("button", { name: "Save" }) as HTMLButtonElement;
+    expect(btn.disabled).toBe(true);
+    expect(btn.getAttribute("aria-busy")).toBe("true");
+    // the leading icon is swapped for a spinner
+    expect(btn.querySelector("[data-testid=ic]")).toBeNull();
+    fireEvent.click(btn);
+    expect(onClick).not.toHaveBeenCalled();
+  });
+
+  it("loading disables even when an explicit disabled={false} is passed", () => {
+    // Regression: `disabled ?? loading` let an explicit boolean disabled shadow loading, so a
+    // call-site with `disabled={!valid} loading={pending}` (valid → disabled=false) stayed
+    // clickable mid-mutation and double-submitted. The contract is: loading always disables.
+    const onClick = vi.fn();
+    render(
+      <Button disabled={false} loading onClick={onClick}>
+        Create
+      </Button>,
+    );
+    const btn = screen.getByRole("button", { name: "Create" }) as HTMLButtonElement;
+    expect(btn.disabled).toBe(true);
+    fireEvent.click(btn);
+    expect(onClick).not.toHaveBeenCalled();
+  });
+
+  it("fullWidth adds the w-full utility", () => {
+    render(<Button fullWidth>Wide</Button>);
+    expect(screen.getByRole("button", { name: "Wide" }).className).toContain("w-full");
+  });
+});
diff --git a/frontend/src/components/ui/Button.tsx b/frontend/src/components/ui/Button.tsx
new file mode 100644
index 00000000..12b44960
--- /dev/null
+++ b/frontend/src/components/ui/Button.tsx
@@ -0,0 +1,103 @@
+import { forwardRef } from "react";
+import type { ButtonHTMLAttributes, ReactNode } from "react";
+import clsx from "clsx";
+import { Loader2 } from "lucide-react";
+
+// The ONE action-button primitive. Before this there were 385 raw <button>s styling themselves with
+// ad-hoc utility strings over the CSS paint-recipes (.btn-primary/.btn-danger, index.css) + the btnCls
+// constant. This centralises the geometry+paint so an "action button" (submit/cancel/subscribe/delete…)
+// is one component with named variants. It deliberately does NOT cover the app's OTHER button families,
+// which keep their own primitives: aria-pressed segmented pills (SegmentedToggle/SortControl), the
+// Switch toggle, .chip removers, dropdown/popover trigger pills, and slider/overlay/card-click buttons.
+//
+// No focus-visible here on purpose: @layer base in index.css already rings every <button> for keyboard
+// users. Adding one would double the ring.
+
+type ButtonVariant =
+  | "primary" // solid accent — the main affirmative action
+  | "danger" // solid danger — destructive confirm
+  | "outline" // bordered, quiet — secondary action (Full history, Subscribe, Cancel-with-border)
+  | "ghost" // borderless, hover-fill — tertiary/dismiss (dialog Cancel, close)
+  | "glass" // glass card — the Plex/overlay action buttons
+  | "danger-outline"; // bordered danger — a destructive action that shouldn't shout
+
+type ButtonSize =
+  | "xs" // dense inline action
+  | "sm" // the btnCls geometry (px-3 py-1.5)
+  | "md" // default action button (px-3 py-2)
+  | "lg" // prominent CTA
+  | "icon"; // square icon-only (w-9 h-9)
+
+const VARIANTS: Record<ButtonVariant, string> = {
+  // .btn-primary / .btn-danger own the fill + one hover-dim (index.css @layer components).
+  primary: "btn-primary",
+  danger: "btn-danger",
+  outline: "border border-border text-muted hover:text-fg hover:border-fg/40",
+  ghost: "text-muted hover:text-fg hover:bg-surface",
+  glass: "glass-card glass-hover hover:text-accent",
+  "danger-outline": "border border-danger/60 text-danger hover:bg-danger/10 hover:border-danger",
+};
+
+const SIZES: Record<ButtonSize, string> = {
+  xs: "px-2 py-1 text-xs rounded-md gap-1",
+  sm: "px-3 py-1.5 text-sm rounded-lg gap-1.5",
+  md: "px-3 py-2 text-sm rounded-lg gap-1.5",
+  lg: "px-4 py-2.5 text-base rounded-lg gap-2",
+  icon: "w-9 h-9 rounded-lg justify-center",
+};
+
+const BASE =
+  "inline-flex items-center justify-center font-medium transition disabled:opacity-50 disabled:cursor-not-allowed";
+
+export interface ButtonProps extends ButtonHTMLAttributes<HTMLButtonElement> {
+  variant?: ButtonVariant;
+  size?: ButtonSize;
+  /** Leading icon (an lucide element). Replaced by a spinner while `loading`. */
+  icon?: ReactNode;
+  /** Render only the icon: children are dropped and `label` supplies the accessible name. */
+  iconOnly?: boolean;
+  /** Accessible name + tooltip — required for `iconOnly`, optional otherwise. */
+  label?: string;
+  /** Disable + swap the icon for a spinner + mark aria-busy. */
+  loading?: boolean;
+  fullWidth?: boolean;
+}
+
+/** The shared action button. `type` defaults to "button" so it never accidentally submits a form. */
+export const Button = forwardRef<HTMLButtonElement, ButtonProps>(function Button(
+  {
+    variant = "primary",
+    size = "md",
+    icon,
+    iconOnly = false,
+    label,
+    loading = false,
+    fullWidth = false,
+    className,
+    children,
+    disabled,
+    type,
+    ...rest
+  },
+  ref,
+) {
+  const leading = loading ? <Loader2 className="w-4 h-4 animate-spin" /> : icon;
+  return (
+    <button
+      ref={ref}
+      type={type ?? "button"}
+      // `loading` must always disable (see the prop's contract above), even when a call-site also
+      // passes an explicit boolean `disabled` — so `||`, not `??`. With `??`, `disabled={false}`
+      // would shadow `loading` and leave the button clickable mid-mutation (double-submit).
+      disabled={disabled || loading}
+      aria-busy={loading || undefined}
+      aria-label={label}
+      title={label}
+      className={clsx(BASE, VARIANTS[variant], SIZES[size], fullWidth && "w-full", className)}
+      {...rest}
+    >
+      {leading}
+      {!iconOnly && children}
+    </button>
+  );
+});
diff --git a/frontend/src/components/ui/DraftSaveBar.tsx b/frontend/src/components/ui/DraftSaveBar.tsx
index 2dd3dd5a..0cf39a5f 100644
--- a/frontend/src/components/ui/DraftSaveBar.tsx
+++ b/frontend/src/components/ui/DraftSaveBar.tsx
@@ -1,5 +1,6 @@
 import { createPortal } from "react-dom";
 import { Check, RotateCcw, Save } from "lucide-react";
+import { Button } from "./Button";
 
 // The Save / Discard bar shown while an edited draft has unsaved changes (Settings prefs,
 // admin Configuration). Two visual variants share one state machine + markup:
@@ -58,24 +59,27 @@ export function DraftSaveBar({
         )}
       </span>
       <div className="flex items-center gap-2">
-        <button
+        <Button
           data-testid="draft-discard"
+          variant="glass"
+          size="sm"
+          icon={<RotateCcw className="w-4 h-4" />}
           onClick={onDiscard}
           disabled={saving || !dirty}
-          className="glass-card glass-hover flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm disabled:opacity-40 transition"
         >
-          <RotateCcw className="w-4 h-4" />
           {labels.discard}
-        </button>
-        <button
+        </Button>
+        <Button
           data-testid="draft-save"
+          variant="primary"
+          size="sm"
+          icon={<Save className="w-4 h-4" />}
           onClick={onSave}
           disabled={saving || !dirty}
-          className="flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm btn-primary font-medium shadow-md shadow-accent/20 disabled:opacity-40 transition"
+          className="shadow-md shadow-accent/20"
         >
-          <Save className="w-4 h-4" />
           {saving ? labels.saving : labels.save}
-        </button>
+        </Button>
       </div>
     </div>
   );
diff --git a/frontend/src/i18n/locales/en/auditActions.json b/frontend/src/i18n/locales/en/auditActions.json
index a9e8e867..2b9e6a28 100644
--- a/frontend/src/i18n/locales/en/auditActions.json
+++ b/frontend/src/i18n/locales/en/auditActions.json
@@ -17,5 +17,7 @@
   "sync_pause": "Sync paused",
   "sync_resume": "Sync resumed",
   "job_run": "Scheduler job run",
-  "audit_clear": "Audit log cleared"
+  "audit_clear": "Audit log cleared",
+  "download_credential_set": "Download cookies set",
+  "download_credential_delete": "Download cookies removed"
 }
diff --git a/frontend/src/i18n/locales/en/downloads.json b/frontend/src/i18n/locales/en/downloads.json
index ff3f92fd..68762c63 100644
--- a/frontend/src/i18n/locales/en/downloads.json
+++ b/frontend/src/i18n/locales/en/downloads.json
@@ -218,7 +218,11 @@
         },
         "login_required": {
           "short": "Sign-in required",
-          "body": "This video needs a signed-in, age-verified, or members-only account to view, so it can't be downloaded here."
+          "body": "This video needs a signed-in account (age-restricted, members-only, or a private group) to view. Add your cookies for this site under Settings → Account → Download authentication to download it."
+        },
+        "unsupported_host": {
+          "short": "Site not supported",
+          "body": "This site isn't supported for downloading, so nothing could be fetched from this link."
         },
         "geo_blocked": {
           "short": "Region-blocked",
@@ -249,6 +253,11 @@
           "body": "Something went wrong while downloading this item. The technical details below may help."
         }
       }
+    },
+    "cookies": {
+      "empty": "Paste your cookies first.",
+      "format": "That doesn't look like a cookies.txt file (Netscape format).",
+      "generic": "Those cookies couldn't be saved."
     }
   }
 }
diff --git a/frontend/src/i18n/locales/en/playlists.json b/frontend/src/i18n/locales/en/playlists.json
index d10d8771..c9ae9858 100644
--- a/frontend/src/i18n/locales/en/playlists.json
+++ b/frontend/src/i18n/locales/en/playlists.json
@@ -35,8 +35,21 @@
   "pushTitle": "Sync to YouTube",
   "pushConfirm": "Sync",
   "pushPlanCreate": "Create a new YouTube playlist with {{count}} videos? (~{{units}} quota units; {{left}} left today.)",
-  "pushPlanUpdate": "Update on YouTube — add {{insert}}, remove {{del}}, reorder {{reorder}}? (~{{units}} quota units; {{left}} left today.)",
-  "pushDiverged": "{{count}} video(s) currently on YouTube will be removed.",
+  "mergeAddLocal_one": "{{count}} video added to Siftlode from YouTube.",
+  "mergeAddLocal_other": "{{count}} videos added to Siftlode from YouTube.",
+  "mergeRemoveLocal_one": "{{count}} video removed from Siftlode.",
+  "mergeRemoveLocal_other": "{{count}} videos removed from Siftlode.",
+  "mergeAddYt_one": "{{count}} video added to YouTube.",
+  "mergeAddYt_other": "{{count}} videos added to YouTube.",
+  "mergeRemoveYt_one": "{{count}} video removed from YouTube.",
+  "mergeRemoveYt_other": "{{count}} videos removed from YouTube.",
+  "mergeReorder_one": "{{count}} video reordered on YouTube.",
+  "mergeReorder_other": "{{count}} videos reordered on YouTube.",
+  "mergeRename": "The playlist will be renamed on YouTube.",
+  "mergeYtUnchanged": "Your YouTube playlist stays unchanged.",
+  "mergeLocalUnchanged": "Your Siftlode playlist stays unchanged.",
+  "mergeQuota": " (~{{units}} quota units; {{left}} left today.)",
+  "mergeQuotaFree": " (~0 quota units.)",
   "pushNoQuota": "Not enough YouTube quota left today ({{left}}) for this sync (~{{units}} needed). Try again tomorrow.",
   "pushUpToDate": "“{{name}}” is already in sync with YouTube.",
   "pushDone": "“{{name}}” synced to YouTube ✓",
diff --git a/frontend/src/i18n/locales/en/settings.json b/frontend/src/i18n/locales/en/settings.json
index 48edcae7..62cd079d 100644
--- a/frontend/src/i18n/locales/en/settings.json
+++ b/frontend/src/i18n/locales/en/settings.json
@@ -104,6 +104,27 @@
       "failed": "Couldn't sign out other sessions. Please try again."
     }
   },
+  "downloadAuth": {
+    "title": "Download authentication",
+    "intro": "Some sites (e.g. a private Facebook group you belong to) only let logged-in members view a video. Paste your browser cookies for a site here and your downloads from it will use your login. Cookies are encrypted, kept private to your account, and only ever sent to the matching site.",
+    "configured": "Set · updated {{when}}",
+    "notSet": "Not set",
+    "set": "Set cookies",
+    "update": "Update",
+    "remove": "Remove",
+    "save": "Save",
+    "cancel": "Cancel",
+    "saving": "Saving…",
+    "placeholder": "Paste the contents of your cookies.txt (Netscape format) here",
+    "help": "Export a cookies.txt for this site with a browser extension, then paste it here. It's write-only — we never show it back.",
+    "saved": "Cookies saved.",
+    "removed": "Cookies removed.",
+    "failed": "Couldn't save those cookies.",
+    "removeConfirmTitle": "Remove cookies?",
+    "removeConfirmBody": "Downloads from this site will stop using your login.",
+    "removeConfirmAction": "Remove",
+    "loadError": "Couldn't load your download authentication settings."
+  },
   "demo": {
     "title": "Demo access",
     "intro": "Emails here can enter the shared demo account straight from the login page — no Google sign-in. They just type the address into the field and are let in after a moment.",
diff --git a/frontend/src/i18n/locales/hu/auditActions.json b/frontend/src/i18n/locales/hu/auditActions.json
index 9038c325..84ff8e01 100644
--- a/frontend/src/i18n/locales/hu/auditActions.json
+++ b/frontend/src/i18n/locales/hu/auditActions.json
@@ -17,5 +17,7 @@
   "sync_pause": "Szinkron szüneteltetve",
   "sync_resume": "Szinkron folytatva",
   "job_run": "Ütemező job lefutott",
-  "audit_clear": "Napló ürítve"
+  "audit_clear": "Napló ürítve",
+  "download_credential_set": "Letöltési cookie-k beállítva",
+  "download_credential_delete": "Letöltési cookie-k eltávolítva"
 }
diff --git a/frontend/src/i18n/locales/hu/downloads.json b/frontend/src/i18n/locales/hu/downloads.json
index 7bb25d54..37ac260e 100644
--- a/frontend/src/i18n/locales/hu/downloads.json
+++ b/frontend/src/i18n/locales/hu/downloads.json
@@ -218,7 +218,11 @@
         },
         "login_required": {
           "short": "Bejelentkezés szükséges",
-          "body": "Ehhez a videóhoz bejelentkezett, kor-ellenőrzött vagy tagsági fiók kell, ezért itt nem tölthető le."
+          "body": "Ehhez a videóhoz bejelentkezett fiók kell (korhatáros, csak tagoknak szóló vagy zárt csoport). Töltsd fel a cookie-kat ehhez az oldalhoz a Beállítások → Fiók → Letöltés-hitelesítés alatt a letöltéshez."
+        },
+        "unsupported_host": {
+          "short": "Nem támogatott oldal",
+          "body": "Ez az oldal nem támogatott a letöltéshez, ezért erről a linkről semmit sem sikerült lekérni."
         },
         "geo_blocked": {
           "short": "Régió szerint tiltva",
@@ -249,6 +253,11 @@
           "body": "Valami hiba történt az elem letöltése közben. Az alábbi technikai részletek segíthetnek."
         }
       }
+    },
+    "cookies": {
+      "empty": "Előbb illeszd be a cookie-kat.",
+      "format": "Ez nem tűnik cookies.txt fájlnak (Netscape formátum).",
+      "generic": "A cookie-kat nem sikerült menteni."
     }
   }
 }
diff --git a/frontend/src/i18n/locales/hu/playlists.json b/frontend/src/i18n/locales/hu/playlists.json
index 3048f09f..cddb6c19 100644
--- a/frontend/src/i18n/locales/hu/playlists.json
+++ b/frontend/src/i18n/locales/hu/playlists.json
@@ -35,8 +35,21 @@
   "pushTitle": "Szinkron YouTube-ra",
   "pushConfirm": "Szinkron",
   "pushPlanCreate": "Létrehozol egy új YouTube-listát {{count}} videóval? (~{{units}} kvótaegység; ma még {{left}} maradt.)",
-  "pushPlanUpdate": "Frissítés a YouTube-on — {{insert}} hozzáadása, {{del}} eltávolítása, {{reorder}} átrendezése? (~{{units}} kvótaegység; ma még {{left}} maradt.)",
-  "pushDiverged": "{{count}} videó, amely jelenleg a YouTube-on van, el lesz távolítva.",
+  "mergeAddLocal_one": "{{count}} videó bekerül a Siftlode-listádba a YouTube-ról.",
+  "mergeAddLocal_other": "{{count}} videó bekerül a Siftlode-listádba a YouTube-ról.",
+  "mergeRemoveLocal_one": "{{count}} videó törlődik a Siftlode-listádból.",
+  "mergeRemoveLocal_other": "{{count}} videó törlődik a Siftlode-listádból.",
+  "mergeAddYt_one": "{{count}} videó felkerül a YouTube-ra.",
+  "mergeAddYt_other": "{{count}} videó felkerül a YouTube-ra.",
+  "mergeRemoveYt_one": "{{count}} videó törlődik a YouTube-ról.",
+  "mergeRemoveYt_other": "{{count}} videó törlődik a YouTube-ról.",
+  "mergeReorder_one": "{{count}} videó átrendeződik a YouTube-on.",
+  "mergeReorder_other": "{{count}} videó átrendeződik a YouTube-on.",
+  "mergeRename": "A lista átnevezésre kerül a YouTube-on.",
+  "mergeYtUnchanged": "A YouTube-listád nem változik.",
+  "mergeLocalUnchanged": "A Siftlode-listád nem változik.",
+  "mergeQuota": " (~{{units}} kvótaegység; ma még {{left}} maradt.)",
+  "mergeQuotaFree": " (~0 kvótaegység.)",
   "pushNoQuota": "Nincs elég YouTube-kvóta mára ({{left}}) ehhez a szinkronhoz (~{{units}} kell). Próbáld újra holnap.",
   "pushUpToDate": "A(z) „{{name}}” már szinkronban van a YouTube-bal.",
   "pushDone": "„{{name}}” szinkronizálva a YouTube-ra ✓",
diff --git a/frontend/src/i18n/locales/hu/settings.json b/frontend/src/i18n/locales/hu/settings.json
index 7aad5dfb..de07255d 100644
--- a/frontend/src/i18n/locales/hu/settings.json
+++ b/frontend/src/i18n/locales/hu/settings.json
@@ -104,6 +104,27 @@
       "failed": "Nem sikerült kijelentkeztetni a többi munkamenetet. Próbáld újra."
     }
   },
+  "downloadAuth": {
+    "title": "Letöltés-hitelesítés",
+    "intro": "Egyes oldalak (pl. egy zárt Facebook-csoport, aminek tagja vagy) csak bejelentkezett tagoknak mutatják a videót. Illeszd be ide a böngésződ cookie-jait egy oldalhoz, és az onnan induló letöltések a te bejelentkezésedet használják. A cookie-k titkosítva, csak a te fiókodhoz kötve tárolódnak, és kizárólag az adott oldalnak küldjük el őket.",
+    "configured": "Beállítva · frissítve: {{when}}",
+    "notSet": "Nincs beállítva",
+    "set": "Cookie-k beállítása",
+    "update": "Frissítés",
+    "remove": "Eltávolítás",
+    "save": "Mentés",
+    "cancel": "Mégse",
+    "saving": "Mentés…",
+    "placeholder": "Illeszd be ide a cookies.txt tartalmát (Netscape formátum)",
+    "help": "Exportálj cookies.txt-t ehhez az oldalhoz egy böngészőbővítménnyel, majd illeszd be ide. Csak írható — sosem mutatjuk vissza.",
+    "saved": "Cookie-k mentve.",
+    "removed": "Cookie-k eltávolítva.",
+    "failed": "A cookie-kat nem sikerült menteni.",
+    "removeConfirmTitle": "Cookie-k eltávolítása?",
+    "removeConfirmBody": "Az erről az oldalról induló letöltések nem használják tovább a bejelentkezésedet.",
+    "removeConfirmAction": "Eltávolítás",
+    "loadError": "Nem sikerült betölteni a letöltési hitelesítési beállításokat."
+  },
   "demo": {
     "title": "Demo hozzáférés",
     "intro": "Az itt szereplő e-mailek a bejelentkező oldalról közvetlenül beléphetnek a közös demo fiókba — Google-bejelentkezés nélkül. Csak beírják a címet a mezőbe, és pár pillanat múlva bekerülnek.",
diff --git a/frontend/src/lib/api/core.ts b/frontend/src/lib/api/core.ts
index 9fa40cb2..82e125f5 100644
--- a/frontend/src/lib/api/core.ts
+++ b/frontend/src/lib/api/core.ts
@@ -43,6 +43,8 @@ function localizeDetail(d: unknown): string {
   }
   if (o?.code === "edit")
     return t(`downloads.errors.edit.${o.reason}`, t("downloads.errors.edit.generic"));
+  if (o?.code === "cookies")
+    return t(`downloads.errors.cookies.${o.reason}`, t("downloads.errors.cookies.generic"));
   return t("errors.generic");
 }
 
diff --git a/frontend/src/lib/api/downloads.ts b/frontend/src/lib/api/downloads.ts
index 25dbf7c9..e53bf088 100644
--- a/frontend/src/lib/api/downloads.ts
+++ b/frontend/src/lib/api/downloads.ts
@@ -137,7 +137,31 @@ export interface AdminDownloadQuota {
   unlimited: boolean;
 }
 
+// Per-user download authentication (cookies). Write-only: the API never returns the stored blob,
+// only which supported hosts are configured + when. See backend downloads.credentials.
+export interface DownloadCredentialHost {
+  host_key: string;
+  label: string;
+  configured: boolean;
+  updated_at: string | null;
+}
+
 export const downloadsApi = {
+  downloadCredentials: (): Promise<{ hosts: DownloadCredentialHost[] }> =>
+    req("/api/downloads/credentials"),
+  setDownloadCredential: (
+    hostKey: string,
+    cookies: string,
+  ): Promise<{ ok: true; hosts: DownloadCredentialHost[] }> =>
+    req(`/api/downloads/credentials/${encodeURIComponent(hostKey)}`, {
+      method: "PUT",
+      body: JSON.stringify({ cookies }),
+    }),
+  deleteDownloadCredential: (
+    hostKey: string,
+  ): Promise<{ ok: true; hosts: DownloadCredentialHost[] }> =>
+    req(`/api/downloads/credentials/${encodeURIComponent(hostKey)}`, { method: "DELETE" }),
+
   downloadProfiles: (): Promise<DownloadProfile[]> => req("/api/downloads/profiles"),
   createDownloadProfile: (p: { name: string; spec: DownloadSpec }): Promise<DownloadProfile> =>
     req("/api/downloads/profiles", { method: "POST", body: JSON.stringify(p) }),
diff --git a/frontend/src/lib/api/playlists.ts b/frontend/src/lib/api/playlists.ts
index 46fb911a..4a085532 100644
--- a/frontend/src/lib/api/playlists.ts
+++ b/frontend/src/lib/api/playlists.ts
@@ -31,12 +31,16 @@ export interface PlaylistDetail {
   items: Video[];
 }
 
+// Two-way sync plan: what each side gains/loses under the 3-way merge (neither side wins
+// wholesale). Local edits are free; only the YouTube writes cost quota.
 export interface PushPlan {
   action: "create" | "update";
-  to_insert: number;
-  to_delete: number;
+  add_yt: number; // local-adds to push out to YouTube
+  add_local: number; // YouTube-adds to pull into Siftlode
+  remove_yt: number; // genuine local deletes propagated to YouTube
+  remove_local: number; // genuine YouTube deletes propagated to Siftlode
   to_reorder: number;
-  yt_extra: number; // items on YouTube that the push would remove (divergence)
+  rename: boolean;
   units_estimate: number;
   remaining_today: number;
   affordable: boolean;
@@ -44,9 +48,11 @@ export interface PushPlan {
 
 export interface PushResult {
   created: number;
-  inserted: number;
-  deleted: number;
+  inserted: number; // added on YouTube
+  deleted: number; // removed on YouTube
   reordered: number;
+  pulled: number; // added into Siftlode from YouTube
+  removed_local: number; // removed from Siftlode (deleted on YouTube)
   failures: string[];
   yt_playlist_id: string | null;
 }
diff --git a/frontend/src/lib/queryKeys.ts b/frontend/src/lib/queryKeys.ts
index e690c564..58adb1d7 100644
--- a/frontend/src/lib/queryKeys.ts
+++ b/frontend/src/lib/queryKeys.ts
@@ -78,6 +78,7 @@ export const qk = {
   downloadIndex: () => ["download-index"] as const,
   downloadUsage: () => ["download-usage"] as const,
   downloadProfiles: () => ["download-profiles"] as const,
+  downloadCredentials: () => ["download-credentials"] as const,
   downloadLinks: (jobId: number) => ["download-links", jobId] as const,
   dlQuota: (userId: number) => ["dl-quota", userId] as const,
   storyboard: (jobId: number) => ["storyboard", jobId] as const,
diff --git a/frontend/src/lib/releaseNotes.ts b/frontend/src/lib/releaseNotes.ts
index 1251a6bb..48be328b 100644
--- a/frontend/src/lib/releaseNotes.ts
+++ b/frontend/src/lib/releaseNotes.ts
@@ -14,6 +14,26 @@ export interface ReleaseEntry {
 }
 
 export const RELEASE_NOTES: ReleaseEntry[] = [
+  {
+    version: "0.75.0",
+    date: "2026-08-29",
+    summary:
+      "A grab-bag batch across Plex, downloads and playlists: your watch state stays truer to Plex, shared audio plays on iPad, downloads from a site you're logged into work with your own cookies, and syncing a playlist to YouTube is now a true two-way merge with a preview.",
+    features: [
+      "Syncing a playlist to YouTube is now a true two-way merge: a video you added on YouTube and one you added in Siftlode both survive, genuine removals carry across, and you always see a preview of exactly what will change on each side before anything is written.",
+      "Download from a site that only shows a video to logged-in members (like a private Facebook group you belong to): paste your own browser cookies for that site under Settings → Account, and downloads from it use your login. Cookies are encrypted, kept private to your account, and only ever sent to the matching site.",
+      "Shared audio files (mp3) now play on iPhone and iPad, in a proper audio player instead of a broken video box.",
+    ],
+    fixes: [
+      'A movie you finish on Plex no longer lingers as "in progress" in Siftlode until the nightly reconcile — scrubbing through the end credits now marks it watched within the next sync.',
+      "A download that fails because the site needs a login you haven't set up now says so clearly and doesn't offer a pointless Retry; a genuinely temporary failure still offers one.",
+      "The Plex sort control's direction toggle no longer wraps onto two lines on the narrower popover, so it matches the feed's.",
+      "Adding a download no longer makes the app feel sluggish while it's converted in the background — the conversion is de-prioritised so the rest of the app and your share links stay responsive.",
+    ],
+    chores: [
+      "Introduced one shared action-button primitive across the app (the channel Subscribe button now matches Full history), and put the whole batch through several multi-agent deep-review rounds — findings fixed and re-reviewed each round until it came back clean, including data-integrity fixes to the new playlist merge.",
+    ],
+  },
   {
     version: "0.74.0",
     date: "2026-08-28",