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/Entities/Track.cs

@@ -1,6 +1,5 @@
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
-using System.Diagnostics.CodeAnalysis;
 using Musify.Domain.Abstractions;
 using Musify.Domain.ValueObjects;
 
@@ -31,47 +30,19 @@ namespace Musify.Domain.Entities
         [MaxLength(16)]
         public string? ExternalId { get; set; }
 
-        [Required]
-        public bool DownloadRequested { get; set; } = false;
-
-        [MaxLength(64)]
-        public string? OriginalPictureName { get; set; }
-
-        [MaxLength(64)]
-        public string? SmallPictureName { get; set; }
-
-        [MaxLength(64)]
-        public string? MediumPictureName { get; set; }
-
-        [MaxLength(64)]
-        public string? LargePictureName { get; set; }
-
-        [MaxLength(64)]
-        public string? OriginalAudioName { get; set; }
-
-        [MaxLength(64)]
-        public string? AudioFolderName { get; set; }
-
-        [Required]
-        public ProcessingStatus PicturesProcessingStatus { get; set; } = ProcessingStatus.Pending;
-
-        [Required]
-        public ProcessingStatus AudioTranscodeProcessingStatus { get; set; } = ProcessingStatus.Pending;
-
         [Required]
         public LifeCycleStatus LifeCycleStatus { get; set; } = LifeCycleStatus.Active;
 
-        [Required]
-        public int RetryCount { get; set; } = 0;
-
-        public DateTime? LastRetryAt { get; set; }
-
         [Required]
         public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
 
         [Required]
         public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
 
+        public TrackPictures Pictures { get; set; } = new();
+
+        public TrackAudio Audio { get; set; } = new();
+
         [ForeignKey(nameof(OwnerUserId))]
         public User? Owner { get; set; }
 
@@ -82,15 +53,5 @@ namespace Musify.Domain.Entities
         public ICollection<ListeningHistory> ListeningHistories { get; set; } = new List<ListeningHistory>();
 
         public ICollection<TrackArtist> TrackArtists { get; set; } = new List<TrackArtist>();
-
-        [NotMapped]
-        [MemberNotNullWhen(true, nameof(SmallPictureName)) ]
-        [MemberNotNullWhen(true, nameof(MediumPictureName)) ]
-        [MemberNotNullWhen(true, nameof(LargePictureName)) ]
-        public bool IsPicturesProcessed => PicturesProcessingStatus == ProcessingStatus.Completed;
-
-        [NotMapped]
-        [MemberNotNullWhen(true, nameof(AudioFolderName)) ]
-        public bool IsAudioProcessed => AudioTranscodeProcessingStatus == ProcessingStatus.Completed;
     }
 }