feat(downloads): clickable channels + poster fallback for thumbnail-less sources

Two visual gaps for non-catalog downloads:
- Channel link: YouTube already exposes channel_url; Facebook exposes none but a
  numeric uploader_id that resolves at facebook.com/<id>. `_uploader_url` derives
  it so the auto-detected channel renders as a real clickable link.
- Poster: a source with no thumbnail (e.g. a direct reddit HLS URL) showed a
  blank image box. The worker now cuts a representative frame with ffmpeg
  (`ensure_poster`) into the `<base>.jpg` sidecar and records `poster_path`
  (migration 0050). The card, the public watch page (<video poster> + og:image),
  and link previews fall back to it via new authed + public poster endpoints.

Adds `app.downloads.backfill` (one-off, re-run-safe) to fill uploader_url
(re-extract YouTube/Facebook metadata) and posters for pre-existing downloads.

backend/app/models.py

@@ -722,6 +722,9 @@ class MediaAsset(Base, TimestampMixin):
     )  # "pending" | "ready" | "error"
     rel_path: Mapped[str | None] = mapped_column(Text)  # relative to download_root
     storyboard_path: Mapped[str | None] = mapped_column(Text)  # phase-2 filmstrip mosaic
+    # A locally-generated poster frame (ffmpeg), set only when the source had NO thumbnail so the
+    # card / watch page / link preview can still show an image. Relative to download_root.
+    poster_path: Mapped[str | None] = mapped_column(Text)
     size_bytes: Mapped[int | None] = mapped_column(BigInteger)
     duration_s: Mapped[int | None] = mapped_column(Integer)
     width: Mapped[int | None] = mapped_column(Integer)