feat(downloads): per-user, per-host yt-dlp cookie/auth support (S6)

Each user stores their own login cookies (Netscape cookies.txt) for an
auth-walled download host (Facebook, Instagram, X, TikTok, Reddit) in
Settings; a download of an auth-walled link on that host is retried with
the acting user's cookie.

Two load-bearing security controls, both in downloads.credentials as the
single source of truth so enqueue and the worker can't drift:
- cache privacy: a cookie-authed download salts its MediaAsset format_sig
  with the user id, so the authed asset is private and never cache-served
  to another user. The worker attaches a cookie ONLY when the asset it is
  downloading into IS that private, salted identity.
- host scope (SEC-12): a cookie is resolved only for a URL whose netloc
  the credential owns (netloc-scoped _host_is); a look-alike/unrelated
  host gets none.

cookies_enc is Fernet-encrypted at rest, only decrypted in-worker into a
0600 file under staging/.auth/ (survives the retry wipe + _produced_file
scan) and unlinked in finally. The blob is never returned by a route,
logged, audited, or written to a job's error/snapshot. cookiefile is set
after the adapter's extra merge so an adapter can't clobber it. Settings
UI is write-only (host + set/updated only, never echoed). New
download_credentials table + migration 0074; per-user routes; HU+EN i18n.

Tests: host-scope, cache-privacy (no cross-user serve), encrypt-at-rest,
write-only API, worker attach-gate; FE dom test for the write-only field.

backend/app/models.py

@@ -867,6 +867,35 @@ 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).