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/TrackPictures.cs

@@ -0,0 +1,23 @@
+using System.Diagnostics.CodeAnalysis;
+
+namespace Musify.Domain.ValueObjects
+{
+#pragma warning disable CS8618
+    public class TrackPictures
+    {
+        public string? OriginalName { get; set; }
+
+        public string? SmallName { get; set; }
+
+        public string? MediumName { get; set; }
+
+        public string? LargeName { get; set; }
+
+        public ProcessingStatus ProcessingStatus { get; set; } = ProcessingStatus.Pending;
+
+        [MemberNotNullWhen(true, nameof(SmallName))]
+        [MemberNotNullWhen(true, nameof(MediumName))]
+        [MemberNotNullWhen(true, nameof(LargeName))]
+        public bool IsProcessed => ProcessingStatus == ProcessingStatus.Completed;
+    }
+}