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/app.dart

@@ -6,6 +6,7 @@ import 'cache/metadata_prefetcher.dart';
 import 'cache/mutation_queue.dart';
 import 'cache/prefetcher.dart';
 import 'cache/sync_controller.dart';
+import 'player/play_events_reporter.dart';
 import 'shared/live_events_dispatcher.dart';
 import 'shared/routing.dart';
 import 'theme/theme_data.dart';
@@ -55,6 +56,11 @@ class _MinstrelAppState extends ConsumerState<MinstrelApp> {
       // enqueue on REST failure so user intent persists across
       // network loss instead of getting rolled back.
       ref.read(mutationReplayerProvider);
+      // Play-events reporter (#415): the Flutter client otherwise
+      // reports no plays at all — this feeds history, recommendation
+      // scoring, scrobbles, and system-playlist rotation, and is the
+      // path that carries the `source` tag for #415.
+      ref.read(playEventsReporterProvider);
     });
   }