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/downloads/og.py

@@ -50,7 +50,12 @@ def _watch_tags(token: str, db: Session) -> str | None:
         return None
     title = (job.display_name or asset.title or "Video").strip()
     channel = (job.display_uploader or asset.uploader or "").strip()
-    image = asset.thumbnail_url  # already an absolute (youtube/fb) URL, or None
+    # Prefer the source thumbnail; fall back to our generated poster (absolute, token-gated URL) so
+    # even a thumbnail-less video (e.g. a reddit clip) unfurls with an image. (This block only runs
+    # for non-password links, so the poster needs no grant.)
+    image = asset.thumbnail_url or (
+        f"{settings.app_base}/api/public/watch/{token}/poster.jpg" if asset.poster_path else None
+    )
     tags = [
         f"<title>{html.escape(title)}</title>",
         _tag("og:title", title),