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.Infrastructure/MassTransit/Activities/Audio/UpdateTrackAudioActivity.cs

@@ -52,12 +52,12 @@ namespace Musify.Infrastructure.MassTransit.Activities.Audio
 
                 var log = new UpdateTrackAudioLog(
                     track.Id,
-                    track.AudioFolderName,
+                    track.Audio.FolderName,
                     track.Duration);
 
-                track.AudioFolderName = Path.GetFileName(audioFolderKey);
+                track.Audio.FolderName = Path.GetFileName(audioFolderKey);
                 track.Duration = durationSeconds;
-                track.AudioTranscodeProcessingStatus = ProcessingStatus.Completed;
+                track.Audio.TranscodeStatus = ProcessingStatus.Completed;
                 await database.SaveChangesAsync(executeContext.CancellationToken);
 
                 logger.LogInformation("Updated track {TrackId} audio", executeContext.Arguments.TrackId);
@@ -75,7 +75,7 @@ namespace Musify.Infrastructure.MassTransit.Activities.Audio
 
                 try
                 {
-                    track.AudioTranscodeProcessingStatus = ProcessingStatus.Failed;
+                    track.Audio.TranscodeStatus = ProcessingStatus.Failed;
                     await database.SaveChangesAsync(executeContext.CancellationToken);
                 }
                 catch (Exception dbException)
@@ -96,9 +96,9 @@ namespace Musify.Infrastructure.MassTransit.Activities.Audio
                 if (track is null)
                     throw new InvalidOperationException($"Track with id {compensateContext.Log.TrackId} not found");
 
-                track.AudioFolderName = compensateContext.Log.PreviousAudioFolderName;
+                track.Audio.FolderName = compensateContext.Log.PreviousAudioFolderName;
                 track.Duration = compensateContext.Log.PreviousDuration;
-                track.AudioTranscodeProcessingStatus = ProcessingStatus.Failed;
+                track.Audio.TranscodeStatus = ProcessingStatus.Failed;
                 await database.SaveChangesAsync(compensateContext.CancellationToken);
 
                 logger.LogInformation("Compensated track {TrackId} audio", compensateContext.Log.TrackId);