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/GetTrackStream.cs

@@ -22,20 +22,20 @@ namespace Musify.Application.Tracks
         {
             var track = await database.Tracks.AsNoTracking()
                 .Where(t => t.Id == request.TrackId)
-                .Select(t => new { t.Id, t.AudioTranscodeProcessingStatus, t.AudioFolderName } )
+                .Select(t => new { t.Id, t.Audio.TranscodeStatus, t.Audio.FolderName } )
                 .SingleOrDefaultAsync(cancellationToken);
 
             if (track == null)
                 return Error.NotFound();
 
-            if (string.IsNullOrWhiteSpace(track.AudioFolderName)
-                || track.AudioTranscodeProcessingStatus != ProcessingStatus.Completed)
+            if (string.IsNullOrWhiteSpace(track.FolderName)
+                || track.TranscodeStatus != ProcessingStatus.Completed)
             {
                 logger.LogInformation("Stream requested for track {TrackId} but audio is not ready", request.TrackId);
                 return Error.Conflict(description: "Track audio is not available for streaming yet.");
             }
 
-            var response = await streamIssuer.IssueAsync(track.Id, track.AudioFolderName, request.UserId, cancellationToken);
+            var response = await streamIssuer.IssueAsync(track.Id, track.FolderName, request.UserId, cancellationToken);
 
             logger.LogInformation("Issued stream ticket for track {TrackId} to user {UserId}", request.TrackId, request.UserId);