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/AlbumHasTrack.cs

@@ -0,0 +1,30 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Musify.Domain.Entities
+{
+#pragma warning disable CS8618
+    [Table("AlbumHasTrack")]
+    public class AlbumHasTrack
+    {
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required]
+        public required Guid AlbumId { get; set; }
+
+        [Required]
+        public required Guid TrackId { get; set; }
+
+        [Required]
+        public int TrackNumber { get; set; } = 1;
+
+        [Required]
+        public DateTime AddedAt { get; set; } = DateTime.UtcNow;
+
+        [ForeignKey(nameof(AlbumId))]
+        public Album Album { get; set; }
+
+        [ForeignKey(nameof(TrackId))]
+        public Track Track { get; set; }
+    }
+}