Three new tables (general_likes, general_likes_albums,
general_likes_artists), seven native /api/likes/* endpoints, four
Subsonic handlers (star/unstar/getStarred/getStarred2), and web UI
heart buttons on TrackRow/AlbumCard/ArtistRow/PlayerBar plus a new
/library/liked page. Closes M2 (with the events foundation already
in main).
Liked state is computed client-side via O(1) Set lookups against a
single createLikedIdsQuery cache; server stays simple per request
(no joins added to entity reads). Optimistic updates with rollback.
