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/Responses/TrackApplicationResponse.cs

@@ -24,7 +24,7 @@ namespace Musify.Application.Tracks.Responses
                 FormatArtist(track),
                 track.Source,
                 track.ExternalId,
-                track.AudioTranscodeProcessingStatus,
+                track.Audio.TranscodeStatus,
                 track.Duration,
                 listensCount,
                 track.OwnerUserId,