Fix playlist thumbnails — extract from yt-dlp thumbnails array

_stable_thumbnail expects a video ID but was being passed a playlist ID
(PLxxx), producing a broken URL. Now picks the best thumbnail from
yt-dlp's thumbnails array, falling back to the singular thumbnail field.

Also backfills playlist.thumbnail_url from the first video when indexing
a playlist that still has no thumbnail.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/playlists.py

@@ -189,6 +189,10 @@ def _index_playlist_task(playlist_id: int, youtube_playlist_id: str, channel_id:
         playlist.video_count = len(video_yt_ids)
         playlist.indexed_at = datetime.utcnow()
         playlist.video_ids = json.dumps(video_yt_ids)
+        # Backfill thumbnail from first video if playlist has none
+        if not playlist.thumbnail_url and video_yt_ids:
+            from ..services.ytdlp import _stable_thumbnail
+            playlist.thumbnail_url = _stable_thumbnail(video_yt_ids[0])
         db.commit()
     except Exception:
         db.rollback()