feat(backend): model albums in the domain

Album is abstract with UserAlbum as its first table-per-type child, the same
split Track already uses, so the YouTube albums can join later without moving
anything. Tracks are linked through AlbumHasTrack rather than a column on the
track itself: an external track is shared between users and the same song
shows up in more than one album.

backend/Musify.Domain/Entities/Album.cs

@@ -0,0 +1,39 @@
+using System.ComponentModel.DataAnnotations;
+using Musify.Domain.Abstractions;
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Domain.Entities
+{
+#pragma warning disable CS8618
+    public abstract class Album : IAuditable
+    {
+        [Key]
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required]
+        [MaxLength(200)]
+        public required string Title { get; set; }
+
+        [Required]
+        [MaxLength(200)]
+        public required string NormalizedTitle { get; set; }
+
+        [MaxLength(256)]
+        public string? Description { get; set; }
+
+        public int? ReleaseYear { get; set; }
+
+        public AlbumPictures? Pictures { get; set; }
+
+        [Required]
+        public LifeCycleStatus LifeCycleStatus { get; set; } = LifeCycleStatus.Active;
+
+        [Required]
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+        [Required]
+        public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+
+        public ICollection<AlbumHasTrack> AlbumTracks { get; set; } = new List<AlbumHasTrack>();
+    }
+}