feat(web): events dispatcher posts to /api/events on player transitions

useEventsDispatcher subscribes to player.current/state via \$effect:
on first transition to playing for a new track, POSTs play_started
and caches the id; on track-id change while a play is open, closes
the prior row as play_skipped; on natural completion (state
playing→paused with position >= duration > 0), POSTs play_ended;
on pagehide, navigator.sendBeacon fires a final play_skipped.
Stable per-tab client_id from sessionStorage.

web/src/lib/api/types.ts

@@ -56,3 +56,11 @@ export type SearchResponse = {
 export type RadioResponse = {
   tracks: TrackRef[];
 };
+
+export type EventRequest =
+  | { type: 'play_started';  track_id: string; client_id?: string }
+  | { type: 'play_ended';    play_event_id: string; duration_played_ms: number }
+  | { type: 'play_skipped';  play_event_id: string; position_ms: number };
+
+export type PlayStartedResponse = { play_event_id: string; session_id: string };
+export type EventOkResponse = { ok: true };