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.Domain/ValueObjects/TrackAudio.cs

@@ -0,0 +1,23 @@
+using System.Diagnostics.CodeAnalysis;
+
+namespace Musify.Domain.ValueObjects
+{
+#pragma warning disable CS8618
+    public class TrackAudio
+    {
+        public string? OriginalName { get; set; }
+
+        public string? FolderName { get; set; }
+
+        public ProcessingStatus TranscodeStatus { get; set; } = ProcessingStatus.Pending;
+
+        public bool DownloadRequested { get; set; }
+
+        public int RetryCount { get; set; }
+
+        public DateTime? LastRetryAt { get; set; }
+
+        [MemberNotNullWhen(true, nameof(FolderName))]
+        public bool IsProcessed => TranscodeStatus == ProcessingStatus.Completed;
+    }
+}