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/YouTube/ResolveYouTubeTrackStream.cs

@@ -24,9 +24,9 @@ namespace Musify.Application.YouTube
         {
             var track = await provisioner.FindAsync(request.VideoId, cancellationToken);
 
-            if (track is not null && track.IsAudioProcessed)
+            if (track is not null && track.Audio.IsProcessed)
             {
-                var serverStream = await streamIssuer.IssueAsync(track.Id, track.AudioFolderName, request.UserId, cancellationToken);
+                var serverStream = await streamIssuer.IssueAsync(track.Id, track.Audio.FolderName, request.UserId, cancellationToken);
                 return new YouTubeStreamResponse(
                     YouTubeStreamMode.Server,
                     serverStream.ManifestUrl,