Enrich playlist video dates after indexing

flat-playlist mode returns timestamp=null for most playlist entries so
published_at is missing after the initial index. Now kicks off
_enrich_missing_task (scoped to the playlist size) as a daemon thread
immediately after indexing commits, filling in dates and view counts
in the background via individual video fetches.

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

backend/routers/playlists.py

@@ -204,12 +204,17 @@ 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()
+        return
     finally:
         db.close()
+
+    # Enrich dates and view counts for videos missing them — runs in background
+    import threading
+    from ..routers.channels import _enrich_missing_task
+    threading.Thread(target=_enrich_missing_task, args=(len(video_yt_ids),), daemon=True).start()