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/UserAlbum.cs

@@ -0,0 +1,15 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Musify.Domain.Entities
+{
+#pragma warning disable CS8618
+    public class UserAlbum : Album
+    {
+        [Required]
+        public required long OwnerUserId { get; set; }
+
+        [ForeignKey(nameof(OwnerUserId))]
+        public User Owner { get; set; }
+    }
+}