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/GenerateAudioWorkflowPathsActivity.cs

@@ -32,7 +32,7 @@ namespace Musify.Infrastructure.MassTransit.Activities.Audio
 
             try
             {
-                track.AudioTranscodeProcessingStatus = ProcessingStatus.Processing;
+                track.Audio.TranscodeStatus = ProcessingStatus.Processing;
                 var folderName = Guid.NewGuid().ToString();
                 var sourceFileName = Path.GetFileName(executeContext.Arguments.SourceKey);
                 var workingDirectory = Path.Combine(executeContext.Arguments.TemporaryRootDirectory, folderName);
@@ -57,7 +57,7 @@ namespace Musify.Infrastructure.MassTransit.Activities.Audio
 
                 try
                 {
-                    track.AudioTranscodeProcessingStatus = ProcessingStatus.Failed;
+                    track.Audio.TranscodeStatus = ProcessingStatus.Failed;
                     await database.SaveChangesAsync(executeContext.CancellationToken);
                 }
                 catch (Exception dbException)