fix(backend): compute track listens count via SQL instead of an unloaded navigation

TrackApplicationResponse.FromEntity counted track.ListeningHistories,
but none of the queries that built a response ever Included it, so
the collection was always empty and listensCount was always 0. Each
query now projects the count as a correlated subquery before mapping
to the response.

backend/Musify.Application/Tracks/CreateTrack.cs

@@ -123,7 +123,7 @@ namespace Musify.Application.Tracks
                 return Error.Failure(description: $"Failed to create track for {request.Title}");
             }
 
-            return TrackApplicationResponse.FromEntity(trackEntity);
+            return TrackApplicationResponse.FromEntity(trackEntity, listensCount: 0);
         }
     }
 }