Playlist().Tracks returns nil when its internal Get fails (for example when the
context is canceled at shutdown), and resolvePlaylist called GetAlbumIDs on it,
panicking with a nil pointer dereference. The artwork drain runs on a bare
goroutine, so the panic killed the whole server.
resolvePlaylist now returns an error when Tracks is nil, and the worker recovers
panics per item: it logs the panic with the item details and stack, and marks
the item as a failed attempt so the rest of the batch still runs.
Fixes#6266
52135913d4