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/playlists/playlist_detail_screen.dart

@@ -212,6 +212,9 @@ class _Body extends ConsumerWidget {
                     ref.read(playerActionsProvider).playTracks(
                           playableRefs,
                           initialIndex: startIdx >= 0 ? startIdx : 0,
+                          source: detail.playlist.isSystem
+                              ? detail.playlist.systemVariant
+                              : null,
                         );
                     // Keep liveTrack referenced to avoid an unused-variable
                     // warning while we leave hooks for menu wiring later.
@@ -293,7 +296,10 @@ class _Header extends ConsumerWidget {
             FilledButton.icon(
               onPressed: () {
                 final refs = playable.map(_toTrackRef).toList(growable: false);
-                ref.read(playerActionsProvider).playTracks(refs);
+                ref.read(playerActionsProvider).playTracks(
+                      refs,
+                      source: p.isSystem ? p.systemVariant : null,
+                    );
               },
               icon: const Icon(Icons.play_arrow),
               label: const Text('Play'),

flutter_client/lib/playlists/widgets/playlist_card.dart

@@ -155,7 +155,13 @@ class PlaylistCard extends ConsumerWidget {
   /// index 0. Mirrors the web PlaylistCard's onPlayClick.
   Future<void> _playPlaylist(WidgetRef ref) async {
     final api = await ref.read(playlistsApiProvider.future);
-    final detail = await api.get(playlist.id);
+    // System playlists: fetch the server's rotation-aware order
+    // (#415) and play it as-is, tagged with the source so the
+    // play-events reporter advances that playlist's rotation. User
+    // playlists keep stored order, untagged.
+    final detail = playlist.isSystem
+        ? await api.systemShuffle(playlist.systemVariant!)
+        : await api.get(playlist.id);
     final refs = <TrackRef>[];
     for (final t in detail.tracks) {
       if (t.trackId == null) continue;
@@ -171,13 +177,14 @@ class PlaylistCard extends ConsumerWidget {
       ));
     }
     if (refs.isEmpty) return;
-    // System playlists default to shuffle so replaying within a day
-    // doesn't deliver the identical experience. User playlists keep
-    // stored order.
+    // System playlists already arrive in rotation-aware order from
+    // the server (#415) — play as-is, tagged with the variant so the
+    // reporter advances rotation. No client shuffle. User playlists
+    // play in stored order, untagged.
     await ref.read(playerActionsProvider).playTracks(
           refs,
           initialIndex: 0,
-          shuffle: playlist.isSystem,
+          source: playlist.isSystem ? playlist.systemVariant : null,
         );
   }
 }