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/player/audio_handler.dart

@@ -82,6 +82,15 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
   /// past the fill front needs to rebuild from the stored tracks.
   List<TrackRef> _lastTracks = const [];
 
+  /// #415: which system playlist this queue was seeded from
+  /// ('for_you' | 'discover'), or null for library / user-playlist /
+  /// radio. The play-events reporter reads this so play_started
+  /// carries `source` and the server advances that rotation. A fresh
+  /// setQueueFromTracks from a non-system surface clears it; internal
+  /// rebuilds (skipToQueueItem) preserve it.
+  String? _queueSource;
+  String? get queueSource => _queueSource;
+
   /// Volume stream for UI subscribers. Mirrors the just_audio player's
   /// volume directly; set via setVolume(double).
   Stream<double> get volumeStream => _player.volumeStream;
@@ -109,8 +118,13 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
     if (audioCacheManager != null) _audioCacheManager = audioCacheManager;
   }
 
-  Future<void> setQueueFromTracks(List<TrackRef> tracks, {int initialIndex = 0}) async {
+  Future<void> setQueueFromTracks(
+    List<TrackRef> tracks, {
+    int initialIndex = 0,
+    String? source,
+  }) async {
     if (tracks.isEmpty) return;
+    _queueSource = source;
     final clampedInitial = initialIndex.clamp(0, tracks.length - 1);
 
     // Bump the generation FIRST. Any in-flight _fillRemainingSources
@@ -190,7 +204,9 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
   @override
   Future<void> skipToQueueItem(int index) async {
     if (index < 0 || index >= _lastTracks.length) return;
-    await setQueueFromTracks(_lastTracks, initialIndex: index);
+    // Preserve the system-playlist source across an internal rebuild
+    // — a queue-item skip is still playing from the same playlist.
+    await setQueueFromTracks(_lastTracks, initialIndex: index, source: _queueSource);
     await play();
   }