feat(backend): save a YouTube album with all of its tracks

POST /albums/youtube/{albumId} creates the album in your library and links
every song in it, provisioning each one as an external track without queueing
any download. The external id is kept on the album so saving it twice returns
a conflict instead of a duplicate, and albums can now hold external tracks,
not just the ones you uploaded.

backend/Musify.Domain/Entities/UserAlbum.cs

@@ -9,6 +9,9 @@ namespace Musify.Domain.Entities
         [Required]
         public required long OwnerUserId { get; set; }
 
+        [MaxLength(64)]
+        public string? ExternalId { get; set; }
+
         [ForeignKey(nameof(OwnerUserId))]
         public User Owner { get; set; }
     }