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.

frontend/src/components/WatchPage.tsx

@@ -15,6 +15,7 @@ type Meta = {
   duration_s?: number | null;
   allow_download?: boolean;
   file_url?: string;
+  poster_url?: string | null;
 };
 
 // Compact display of a URL: drop the protocol + trailing slash so it fits on one line.
@@ -175,6 +176,7 @@ export default function WatchPage() {
                 controls
                 playsInline
                 src={meta.file_url}
+                poster={meta.poster_url || undefined}
                 className="max-h-[80vh] max-w-full mx-auto block"
               />
             </div>