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/lib/api.ts

@@ -901,6 +901,7 @@ export interface DownloadJob {
   source_kind: string;
   source_ref: string;
   source_url: string | null; // clean "downloaded from" URL (null for edit clips)
+  poster_url: string | null; // locally-generated poster, fallback when the source had no thumbnail
   profile_id: number | null;
   spec: DownloadSpec;
   display_name: string | null;