fix(jellyfin): hydrate artwork on the song cursor

Jellyfin's listSongs streamed media files via GetCursor, which never
hydrates artwork, so songs emitted entity-id image tags and no blurhash.

media_file now uses the same id pre-pass as the other three cursors
(album/artist/playlist), for consistency, but on a separate method,
GetCursorWithArtwork: GetCursor itself must stay untouched, since it's
also the scanner's hot path and the scanner never reads artwork.

Measured on 1,000,000 tracks, the pre-pass over all ids costs +41.8 MB
heap and +298 ms versus GetCursor's bounded +0.0 MB. The Jellyfin path
is paginated, though, so in practice it only ever pre-passes a page's
worth of ids, not the full library, and doesn't pay that cost.

server/jellyfin/items.go

@@ -588,7 +588,7 @@ func (api *Router) listSongs(ctx context.Context, opts model.QueryOptions, q ite
 	// A full-library request (Finamp's sync, with MediaSources) is tens of thousands of fat rows.
 	total, _ := repo.CountAll(model.QueryOptions{Filters: opts.Filters})
 	open := streamCursor(func() (func(func(model.MediaFile, error) bool), error) {
-		return repo.GetCursor(opts)
+		return repo.GetCursorWithArtwork(opts)
 	}, toItem)
 	return streamed(open, int(total), opts.Offset), nil
 }