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/ValueObjects/AlbumPictures.cs

@@ -0,0 +1,13 @@
+namespace Musify.Domain.ValueObjects
+{
+    public class AlbumPictures
+    {
+        public string? OriginalName { get; set; }
+
+        public string? SmallName { get; set; }
+
+        public string? MediumName { get; set; }
+
+        public string? LargeName { get; set; }
+    }
+}