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.
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.
Admin