fix: check actual thumb.png file instead of has_thumb metadata flag

app.py

@@ -422,7 +422,8 @@ def _rebuild_feed() -> str:
                     with open(meta_path) as f:
                         meta = json.load(f)
                     meta["audio_url"] = f"{BUCKET_URL}/songs/{song_id}/{song_id}.wav"
-                    if meta.get("has_thumb"):
+                    thumb_path = os.path.join(songs_dir, song_id, "thumb.png")
+                    if os.path.isfile(thumb_path):
                         meta["thumb_url"] = f"{BUCKET_URL}/songs/{song_id}/thumb.png"
                     songs.append(meta)
                 except Exception: