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.Application/Contracts/IDatabase.cs

@@ -9,6 +9,12 @@ namespace Musify.Application.Contracts
 
         DbSet<PlayList> PlayLists { get; }
 
+        DbSet<Album> Albums { get; }
+
+        DbSet<UserAlbum> UserAlbums { get; }
+
+        DbSet<AlbumHasTrack> AlbumHasTracks { get; }
+
         DbSet<Track> Tracks { get; }
 
         DbSet<LocalTrack> LocalTracks { get; }