/Artists and /Artists/AlbumArtists both called the same role-agnostic
handler, so Navidrome's per-role artist entries (composers, arrangers,
performers) all showed as Album Artists, and the two tabs were identical.
Filter /Artists/AlbumArtists to RoleAlbumArtist and /Artists to RoleArtist
(and the MusicArtist browse to album artists), via filter.ArtistsByRole.
Verified live: Beatles Singles' album-artists dropped 138->5, composers
excluded, performers (Billy Preston) show only under /Artists.
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.
