feat(downloads): ad-hoc clips cut straight from a YouTube video or a Plex item

POST /api/downloads/clip takes a source (a YouTube id or a Plex rating key), a range of at most
ten minutes and, for Plex, the audio track being heard. The clip becomes a per-user asset in the
library like an editor clip, shareable as soon as it is ready. A YouTube clip fetches only its
range (yt-dlp download_ranges: 3-7 s whatever the video's length); a Plex clip reads the library
file in place. Both then go through one re-encode: frame-exact at the marks, H.264 at most 1080p
with HDR tonemapped to SDR, stereo AAC, no chapter track, .mp4.

Job kinds are a JobKind StrEnum now, and Re-download refuses every kind but a download.

backend/app/downloads/service.py

@@ -16,11 +16,21 @@ from sqlalchemy.orm import Session
 
 from app.config import settings
 from app.downloads import adapters
+from app.downloads import clip as clipmod
 from app.downloads import credentials
 from app.downloads import edit as editmod
 from app.downloads import formats, mediakind, quota, storage
-from app.downloads.status import HOLDING, JobStatus
-from app.models import DISPLAY_TEXT_MAX, Channel, DownloadJob, MediaAsset, Video
+from app.downloads.status import HOLDING, JobKind, JobStatus
+from app.models import (
+    DISPLAY_TEXT_MAX,
+    Channel,
+    DownloadJob,
+    MediaAsset,
+    PlexItem,
+    PlexShow,
+    Video,
+)
+from app.plex import paths as plex_paths
 
 
 class EditError(Exception):
@@ -40,10 +50,13 @@ def source_url(source_kind: str, source_ref: str) -> str:
 def reference_url(job, asset) -> str | None:
     """The clean "downloaded from" URL for a job — shared by the private Downloads page and the public
     watch page so both surfaces always agree. None for edit derivatives (a clip's source is the user's
-    own earlier download, not a web page); else the canonical page URL yt-dlp recorded; else one derived
-    from source_kind+source_ref (covers queued/older rows before the worker fills it)."""
-    if job is None or job.job_kind == "edit":
+    own earlier download, not a web page) and for a Plex clip (the library is not a public page); a
+    YouTube clip links its video at the clip's start; else the canonical page URL yt-dlp recorded; else
+    one derived from source_kind+source_ref (covers queued/older rows before the worker fills it)."""
+    if job is None or job.job_kind == JobKind.EDIT:
         return None
+    if job.job_kind == JobKind.CLIP:
+        return clipmod.reference_url(job.edit_spec or {})
     if asset is not None and asset.source_webpage_url:
         return asset.source_webpage_url
     if job.source_kind in ("youtube", "url"):
@@ -251,7 +264,7 @@ def enqueue_edit(
     job = DownloadJob(
         user_id=user_id,
         asset_id=asset.id,
-        job_kind="edit",
+        job_kind=JobKind.EDIT,
         source_kind="edit",
         source_ref=ref,
         source_job_id=source_job.id,
@@ -275,6 +288,95 @@ def enqueue_edit(
     return job
 
 
+def _clip_source_facts(db: Session, spec: dict) -> dict:
+    """Title, uploader, thumbnail and length of what a clip is cut from, read from our own catalog.
+    Raises ClipError when a Plex item is unknown or its file cannot be read here — said at enqueue,
+    rather than as a job that fails in the queue a minute later."""
+    if spec["source"] == "youtube":
+        video = db.get(Video, spec["ref"])
+        if video is None:  # not in the catalog: the worker fills the title from yt-dlp's answer
+            return {"title": None, "uploader": None, "thumbnail_url": None, "length_s": None}
+        channel = db.get(Channel, video.channel_id)
+        return {
+            "title": video.title,
+            "uploader": channel.title if channel else None,
+            "thumbnail_url": video.thumbnail_url,
+            "length_s": video.duration_seconds,
+        }
+    item = db.execute(
+        select(PlexItem).where(PlexItem.rating_key == spec["ref"])
+    ).scalar_one_or_none()
+    if item is None:
+        raise clipmod.ClipError("not_found")
+    if plex_paths.local_media_path(db, item.file_path) is None:
+        raise clipmod.ClipError("media_missing")
+    title = item.title
+    show = db.get(PlexShow, item.show_id) if item.show_id else None
+    if show is not None:
+        episode = (
+            f"S{item.season_number}·E{item.episode_number}"
+            if item.season_number is not None and item.episode_number is not None
+            else None
+        )
+        title = " — ".join(p for p in (show.title, episode, item.title) if p)
+    return {
+        "title": title,
+        "uploader": None,
+        # Plex art is served through our own authenticated routes, never a public URL; the worker
+        # cuts the clip's poster from the clip itself.
+        "thumbnail_url": None,
+        "length_s": item.duration_s,
+    }
+
+
+def enqueue_clip(
+    db: Session, user_id: int, spec: dict, display_name: str | None = None
+) -> DownloadJob:
+    """Queue an ad-hoc clip (`clipmod.normalize`d spec) for `user_id`: a NEW per-user asset cut
+    straight from the source — a YouTube range or a Plex library file (see downloads/clip).
+
+    Raises ClipError (unknown or unreachable source, a range past its end) or quota.QuotaExceeded."""
+    facts = _clip_source_facts(db, spec)
+    # One second of grace: a mark set on the last frame may round past the catalog's whole seconds.
+    if facts["length_s"] and spec["end_s"] > facts["length_s"] + 1:
+        raise clipmod.ClipError("past_end")
+
+    quota.check_enqueue(db, user_id)
+
+    ref = clipmod.asset_ref(user_id, spec)[:512]
+    asset = get_or_create_asset(db, "clip", ref, clipmod.sig(spec))
+    if not asset.title:
+        asset.title = _clip_title(facts["title"], display_name)
+        asset.uploader = facts["uploader"]
+        asset.thumbnail_url = facts["thumbnail_url"]
+        asset.duration_s = round(clipmod.duration(spec))
+        asset.media_kind = mediakind.VIDEO
+
+    job = DownloadJob(
+        user_id=user_id,
+        asset_id=asset.id,
+        job_kind=JobKind.CLIP,
+        source_kind="clip",
+        source_ref=ref,
+        edit_spec=spec,
+        profile_snapshot={},
+        display_name=display_name or asset.title[:DISPLAY_TEXT_MAX],
+        status="queued",
+        queue_pos=next_queue_pos(db),
+    )
+    db.add(job)
+    incr_ref_count(db, asset.id)
+
+    if asset.status == "ready":  # the same range was already cut for this user → instant
+        job.status = "done"
+        job.progress = 100
+        asset.last_access_at = datetime.now(timezone.utc)
+
+    db.commit()
+    db.refresh(job)
+    return job
+
+
 def finalize_if_running(db: Session, job_id: int, **fields) -> bool:
     """Write terminal `fields` to a job in ONE atomic ``UPDATE ... WHERE status='running'`` and report
     whether it fired. This is the single owner of the invariant "a finishing worker child must never