Fetch first video thumbnail for playlists with no thumbnail

When yt-dlp returns no thumbnail for a playlist entry, fetch the
playlist's first video (max_videos=1) and derive a stable thumbnail
URL from its video ID. Applied during both the initial fetch and
on index (already done on index in previous commit).

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

backend/routers/playlists.py

@@ -68,6 +68,20 @@ def fetch_channel_playlists(
     return {"detail": "Fetching playlists"}
 
 
+def _first_video_thumbnail(youtube_playlist_id: str) -> Optional[str]:
+    """Fetch just the first video of a playlist and return a stable thumbnail URL."""
+    try:
+        videos = ytdlp.fetch_playlist_videos(youtube_playlist_id, max_videos=1)
+        if videos:
+            vid_id = videos[0].get("youtube_video_id")
+            if vid_id:
+                from ..services.ytdlp import _stable_thumbnail
+                return _stable_thumbnail(vid_id)
+    except Exception:
+        pass
+    return None
+
+
 def _fetch_playlists_task(channel_id: int, youtube_channel_id: str):
     from ..database import SessionLocal
     db = SessionLocal()
@@ -75,20 +89,21 @@ def _fetch_playlists_task(channel_id: int, youtube_channel_id: str):
         playlists = ytdlp.fetch_channel_playlists(youtube_channel_id)
         for pl in playlists:
             pl_id = pl["youtube_playlist_id"]
+            thumb = pl.get("thumbnail_url") or _first_video_thumbnail(pl_id)
             existing = db.query(Playlist).filter_by(youtube_playlist_id=pl_id).first()
             if existing:
                 existing.title = pl["title"] or existing.title
                 if pl.get("video_count"):
                     existing.video_count = pl["video_count"]
-                if pl.get("thumbnail_url") and not existing.thumbnail_url:
+                if thumb and not existing.thumbnail_url:
-                    existing.thumbnail_url = pl["thumbnail_url"]
+                    existing.thumbnail_url = thumb
             else:
                 db.add(Playlist(
                     youtube_playlist_id=pl_id,
                     channel_id=channel_id,
                     title=pl["title"],
                     description=pl.get("description"),
-                    thumbnail_url=pl.get("thumbnail_url"),
+                    thumbnail_url=thumb,
                     video_count=pl.get("video_count") or 0,
                 ))
         db.commit()