Specifies POST /api/events with start+end semantics, session service
(30-min rolling), play/skip classification (spec §6 boundaries),
sendBeacon + auto-close-on-next-start for abandoned plays, and
Subsonic /rest/scrobble integration that writes synthetic completed
plays so third-party clients feed the recommendation pipeline.
Schema migration ships the full M2 set (play_events, skip_events,
sessions) including M3-only nullable columns so M3 doesn't need a
follow-up migration. General likes deferred to a separate M2
sub-plan; this slice covers spec §13 step 5 only.
