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/Track.cs

@@ -38,6 +38,8 @@ namespace Musify.Domain.Entities
 
         public ICollection<PlayListHasTrack> PlayListTracks { get; set; } = new List<PlayListHasTrack>();
 
+        public ICollection<AlbumHasTrack> AlbumTracks { get; set; } = new List<AlbumHasTrack>();
+
         public ICollection<ListeningHistory> ListeningHistories { get; set; } = new List<ListeningHistory>();
     }
 }