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/worker.py

@@ -209,6 +209,21 @@ def _finish_from_cache(job_id: int, asset_id: int) -> None:
         db.commit()
 
 
+def _uploader_url(info: dict) -> str | None:
+    """The channel/uploader page URL for the auto-clickable channel line. YouTube gives a
+    `channel_url`/`uploader_url` directly; Facebook gives neither but exposes a numeric
+    `uploader_id` (the page id) that resolves at facebook.com/<id>. Returns None for sources with
+    no derivable channel page (e.g. reddit / a raw media URL)."""
+    url = info.get("channel_url") or info.get("uploader_url")
+    if url:
+        return url
+    uid = info.get("uploader_id")
+    tag = f"{info.get('extractor_key') or info.get('extractor') or ''} {info.get('webpage_url') or ''}".lower()
+    if uid and "facebook" in tag:
+        return f"https://www.facebook.com/{uid}"
+    return None
+
+
 def _fill_asset_meta_early(asset_id: int, info: dict) -> None:
     """Populate an asset's display metadata from yt-dlp's info_dict the first time it's seen
     (only if not already filled at enqueue from our catalog) — so an ad-hoc URL's row shows a
@@ -219,7 +234,7 @@ def _fill_asset_meta_early(asset_id: int, info: dict) -> None:
             return
         asset.title = normalize_title(info.get("title"))
         asset.uploader = info.get("uploader") or info.get("channel")
-        asset.uploader_url = info.get("channel_url") or info.get("uploader_url")
+        asset.uploader_url = _uploader_url(info)
         asset.thumbnail_url = info.get("thumbnail")
         asset.duration_s = int(info["duration"]) if info.get("duration") else None
         asset.upload_date = _parse_upload_date(info.get("upload_date"))
@@ -410,9 +425,13 @@ def _download(job_id: int, asset_id: int, source_kind: str, source_ref: str, spe
                 asset.acodec = info.get("acodec") if info.get("acodec") not in (None, "none") else None
                 asset.title = meta.title
                 asset.uploader = meta.uploader
-                asset.uploader_url = info.get("channel_url") or info.get("uploader_url")
+                asset.uploader_url = _uploader_url(info)
                 asset.upload_date = meta.upload_date
                 asset.thumbnail_url = meta.thumbnail_url
+                # No thumbnail from the source (e.g. a direct media URL) → cut a poster frame from
+                # the file so the card / watch page / link preview still shows an image.
+                if not meta.thumbnail_url:
+                    asset.poster_path = editmod.ensure_poster(asset, settings.download_root)
                 asset.source_webpage_url = info.get("webpage_url")
                 asset.nfo_written = nfo_ok
                 asset.error = None