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/services/ytdlp.py

@@ -378,11 +378,20 @@ def fetch_channel_playlists(channel_id: str, max_results: int = 100) -> list[dic
             title = info.get("title") or info.get("playlist_title") or ""
             if not pl_id or not title or pl_id == channel_id:
                 continue
+            # Thumbnail: yt-dlp gives a thumbnails array for playlist entries;
+            # fall back to singular thumbnail field. Never use _stable_thumbnail
+            # here because the id is a playlist ID, not a video ID.
+            thumbs = info.get("thumbnails") or []
+            thumb_url = info.get("thumbnail")
+            if thumbs:
+                best = max(thumbs, key=lambda t: (t.get("width") or 0) * (t.get("height") or 0), default=None)
+                if best:
+                    thumb_url = best.get("url") or thumb_url
             playlists.append({
                 "youtube_playlist_id": pl_id,
                 "title": title,
                 "description": info.get("description"),
-                "thumbnail_url": _stable_thumbnail(info.get("id")) if info.get("_type") == "url" else None,
+                "thumbnail_url": thumb_url,
                 "video_count": info.get("playlist_count") or info.get("n_entries") or 0,
             })
         except json.JSONDecodeError: