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/edit.py

@@ -298,6 +298,40 @@ def build_storyboard_cmd(src: Path, dest: Path, duration_s: int | None, geom: di
     ]
 
 
+def build_poster_cmd(src: Path, dest: Path, at_s: float) -> list[str]:
+    """Grab one representative frame as a poster JPEG. `-ss` before `-i` seeks fast."""
+    return [
+        "ffmpeg", "-y", "-hide_banner", "-loglevel", "error", "-nostdin",
+        "-ss", f"{max(0.0, at_s):.3f}", "-i", str(src),
+        "-frames:v", "1", "-q:v", "3", "-vf", "scale=640:-2", str(dest),
+    ]
+
+
+def ensure_poster(asset, download_root: str, timeout_s: int = 30) -> str | None:
+    """Generate a poster frame for a ready asset that has no thumbnail (e.g. a direct media URL
+    like a reddit HLS manifest, which yt-dlp can't get a thumbnail for). Written as the media's
+    `<base>.jpg` sidecar — so Plex uses it too and `delete_asset_files` cleans it up — and returned
+    as a rel path (or None on failure). Best-effort + time-bounded; a few seconds in (or 10% of a
+    short clip) dodges black intro frames."""
+    if not asset.rel_path:
+        return None
+    root = Path(download_root)
+    src = root / asset.rel_path
+    if not src.exists():
+        return None
+    rel = str(Path(asset.rel_path).with_suffix(".jpg"))
+    dest = root / rel
+    if dest.exists():
+        return rel
+    at = min(3.0, (asset.duration_s or 10) * 0.1)
+    try:
+        subprocess.run(build_poster_cmd(src, dest, at), capture_output=True, timeout=timeout_s, check=True)
+    except (subprocess.TimeoutExpired, subprocess.CalledProcessError, OSError) as exc:
+        log.info("poster gen skipped for asset %s: %s", asset.id, str(exc)[:120])
+        return None
+    return rel if dest.exists() else None
+
+
 def ensure_storyboard(asset, download_root: str, timeout_s: int = 90) -> dict | None:
     """Return the filmstrip meta for a ready source asset, generating the sprite on first use.