Jellyfin clients (Finamp) send container ids — an album, artist or
playlist — in a playlist's Ids list and expect the server to expand each
into its child tracks. core/playlists only understands media file ids, so
creating or adding with an album id silently produced an empty playlist.
Expand container ids to their tracks (in order) before create/add; bare
song ids still pass through. Adds filter.SongsByArtistID.
Moves server/subsonic/filter to server/filter so server/jellyfin can use
the shared query-option builders without importing server/subsonic,
enforcing the rule that no API package imports another API's package.