refactor(backend): group track pipeline state into owned types

Track mixed catalog fields (Title, Artist credit, Duration) with
picture-processing and audio-transcode pipeline state as ten flat
columns plus two computed [NotMapped] flags, making the entity read
like a media-pipeline worksheet.

Introduce TrackPictures and TrackAudio as EF owned types mapped onto
the same existing columns (explicit column names, no schema change -
verified via `dotnet ef migrations has-pending-model-changes`).
Track now reads as Title/Artist/Duration/Pictures/Audio instead of
fourteen loose properties.

backend/Musify.Application/Tracks/DeleteTrack.cs

@@ -34,8 +34,8 @@ namespace Musify.Application.Tracks
                 return Error.Unauthorized();
             }
 
-            if (track.AudioTranscodeProcessingStatus == ProcessingStatus.Processing
-                || track.AudioTranscodeProcessingStatus == ProcessingStatus.Pending)
+            if (track.Audio.TranscodeStatus == ProcessingStatus.Processing
+                || track.Audio.TranscodeStatus == ProcessingStatus.Pending)
             {
                 logger.LogWarning("Track {TrackId} is currently being processed and cannot be deleted", request.TrackId);
                 return Error.Conflict(description: "Track is currently being processed and cannot be deleted");