send processing album colors to a background thread

- use white color as default album page play button color
- return 404 if album is None on get_album_bio()

server/app/lib/populate.py

@@ -144,7 +144,7 @@ class CreateAlbums:
                 album = create_album(track)
                 self.db_tracks.remove(track)
             else:
-                album["image"] = hash
+                album["image"] = hash + ".webp"
         try:
             album = Album(album)
             return album