feat(flutter): #415 stage 3 — play-events reporter + rotation wiring

The Flutter client previously reported NO plays — mobile listening
never reached play_events, so history, recommendation scoring,
ListenBrainz scrobbles, and #415 rotation all missed mobile entirely.
Operator chose to close that gap properly as part of Stage 3.

New:
- EventsApi (api/endpoints/events.dart): play_started/ended/skipped.
- PlayEventsReporter (player/play_events_reporter.dart): state
  machine over (track id, playing) mirroring the web dispatcher.
  Persists an opaque client_id in secure storage. Deliberate
  divergence from web: a track change inside a queue is classified
  ended-vs-skipped by whether the prior track reached ~its duration
  (3s tolerance), instead of web's blanket "track change = skip"
  which would mark every naturally-finished in-queue track a skip
  and dilute recommendation skip-ratios — the exact failure mode
  that motivated doing this properly. Fail-safe: no-ops when there's
  no audio handler (tests / no-audio env). App-lifecycle paused/
  detached closes an open row as a best-effort skip (web pagehide
  parity). Wired in app.dart postFrame.
- PlaylistsApi.systemShuffle(variant): GET the rotation-aware order.

Wiring:
- audio_handler: _queueSource carried through setQueueFromTracks
  (source param); preserved across internal skipToQueueItem rebuild.
- player_provider.playTracks: source param → setQueueFromTracks.
- PlaylistCard: system playlists fetch systemShuffle and play as-is
  tagged with source (no client shuffle — server already ordered).
- playlist_detail_screen: header Play + per-track tap tag source for
  system playlists so rotation advances from any entry point.

Known/flagged separately: the web dispatcher likely has the same
false-skip-on-advance issue; not fixed here to keep #415 scoped and
clients' wire behavior comparable.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/api/endpoints/playlists.dart

@@ -45,6 +45,19 @@ class PlaylistsApi {
     return PlaylistDetail.fromJson(r.data ?? const {});
   }
 
+  /// GET /api/playlists/system/{variant}/shuffle (#415 stage 2/3).
+  /// Same shape as get() but tracks are server-ordered rotation-aware
+  /// (unplayed-this-rotation first; resets when exhausted). Model
+  /// variant uses underscores; the route segment is hyphenated.
+  /// Intentionally uncached — varies per play.
+  Future<PlaylistDetail> systemShuffle(String variant) async {
+    final seg = variant == 'for_you' ? 'for-you' : variant;
+    final r = await _dio.get<Map<String, dynamic>>(
+      '/api/playlists/system/$seg/shuffle',
+    );
+    return PlaylistDetail.fromJson(r.data ?? const {});
+  }
+
   /// POST /api/playlists/{id}/tracks. Owner only; server returns the
   /// playlist detail with the new rows.
   Future<void> appendTracks(String playlistId, List<String> trackIds) async {