feat(player): resume last session on launch (#54)

The #52 teardown clears the session when idle/dismissed, so the
headset / lock-screen play button had nothing to resume and the user
lost their place.

- track.dart: toJson() (round-trips fromJson)
- db.dart: CachedResumeState single-row snapshot, schema 9->10 + migration
- audio_handler.dart: queuedTracks getter
- player_provider.dart: restoreQueue() — configure + setQueueFromTracks
  + seek, no play (restores PAUSED)
- resume_controller.dart: restores last snapshot paused on launch;
  persists {source,index,position_ms,tracks} debounced on track
  change / pause and immediately on app teardown; never clobbers the
  saved snapshot when the queue is empty so a teardown stays
  recoverable; restore gated on auth + no active session
- app.dart: wired into postFrame

db.g.dart regenerated by CI per project convention. Deferred fast-follow:
media-button-when-fully-stopped re-init (needs a configure()-injected
callback; tracked on #54).

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

flutter_client/lib/cache/db.dart

@@ -230,6 +230,20 @@ class CachedQuarantineMine extends Table {
   Set<Column> get primaryKey => {trackId};
 }
 
+/// Single-row snapshot of the last playback session — queue (TrackRef
+/// JSON), current index, position, and #415 source. Lets a torn-down
+/// session (the #52 idle/dismissed teardown) resume on next launch;
+/// without it the headset / lock-screen play button has nothing to
+/// resume. Mirrors the CachedHomeSnapshot single-row JSON pattern.
+/// Schema 10+.
+class CachedResumeState extends Table {
+  IntColumn get id => integer().withDefault(const Constant(1))();
+  TextColumn get json => text()();
+  DateTimeColumn get updatedAt => dateTime().withDefault(currentDateAndTime)();
+  @override
+  Set<Column> get primaryKey => {id};
+}
+
 enum CacheSource { manual, autoLiked, autoPlaylist, autoPrefetch, incidental }
 
 @DriftDatabase(tables: [
@@ -247,12 +261,13 @@ enum CacheSource { manual, autoLiked, autoPlaylist, autoPrefetch, incidental }
   CachedHomeIndex,
   CachedSystemPlaylistsStatus,
   CachedMutations,
+  CachedResumeState,
 ])
 class AppDb extends _$AppDb {
   AppDb([QueryExecutor? e]) : super(e ?? driftDatabase(name: 'minstrel_cache'));
 
   @override
-  int get schemaVersion => 9;
+  int get schemaVersion => 10;
 
   @override
   MigrationStrategy get migration => MigrationStrategy(
@@ -316,6 +331,12 @@ class AppDb extends _$AppDb {
               'UPDATE audio_cache_index SET last_played_at = cached_at',
             );
           }
+          if (from < 10) {
+            // Schema 10 (#54): cached_resume_state — single-row last-
+            // session snapshot for resume-on-launch. Empty on upgrade;
+            // first persist populates it.
+            await m.createTable(cachedResumeState);
+          }
         },
       );
 }

flutter_client/lib/cache/resume_controller.dart

@@ -0,0 +1,159 @@
+// Resume-on-launch (#54). The #52 teardown tears the audio_service
+// session down (and clears mediaItem) when idle/dismissed, so the
+// headset / lock-screen play button otherwise has nothing to resume.
+// This controller persists the live queue + index + position + #415
+// source to a single-row drift snapshot on track change / pause / app
+// teardown, and on construction restores the last snapshot into the
+// handler PAUSED (the user sees their last track and continues with
+// play / a media button — we never auto-blast on launch).
+//
+// Persist guard: when the handler clears (teardown → mediaItem null /
+// empty queue) we deliberately do NOT write, so the last good snapshot
+// survives for the next launch — that survival is the whole point.
+
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:drift/drift.dart' as drift;
+import 'package:flutter/widgets.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../auth/auth_provider.dart';
+import '../models/track.dart';
+import '../player/player_provider.dart';
+import 'audio_cache_manager.dart' show appDbProvider;
+import 'db.dart';
+
+class ResumeController with WidgetsBindingObserver {
+  ResumeController(this._ref);
+  final Ref _ref;
+
+  final _subs = <StreamSubscription<dynamic>>[];
+  Timer? _debounce;
+  bool _disposed = false;
+
+  Future<void> start() async {
+    try {
+      // audioHandlerProvider throws until main() overrides it (the real
+      // app always does). In tests / no-audio environments there's
+      // nothing to resume — stay inert.
+      _ref.read(audioHandlerProvider);
+    } catch (_) {
+      return;
+    }
+    if (_disposed) return;
+    await _restore();
+    if (_disposed) return;
+    final h = _ref.read(audioHandlerProvider);
+    _subs.add(h.mediaItem.listen((_) => _schedulePersist()));
+    _subs.add(h.playbackState.listen((_) => _schedulePersist()));
+    WidgetsBinding.instance.addObserver(this);
+  }
+
+  void _schedulePersist() {
+    if (_disposed) return;
+    _debounce?.cancel();
+    _debounce = Timer(const Duration(seconds: 3), () {
+      unawaited(_persist());
+    });
+  }
+
+  Future<void> _persist() async {
+    if (_disposed) return;
+    try {
+      final h = _ref.read(audioHandlerProvider);
+      final tracks = h.queuedTracks;
+      final mi = h.mediaItem.value;
+      // Cleared by teardown — keep the last good snapshot for next
+      // launch rather than wiping it with an empty queue.
+      if (tracks.isEmpty || mi == null) return;
+      var idx = tracks.indexWhere((t) => t.id == mi.id);
+      if (idx < 0) idx = 0;
+      final blob = jsonEncode({
+        'v': 1,
+        'source': h.queueSource,
+        'index': idx,
+        'position_ms': h.position.inMilliseconds,
+        'tracks': tracks.map((t) => t.toJson()).toList(),
+      });
+      final db = _ref.read(appDbProvider);
+      await db.into(db.cachedResumeState).insertOnConflictUpdate(
+            CachedResumeStateCompanion.insert(
+              json: blob,
+              updatedAt: drift.Value(DateTime.now()),
+            ),
+          );
+    } catch (e, st) {
+      debugPrint('resume_controller: persist failed: $e\n$st');
+    }
+  }
+
+  Future<void> _restore() async {
+    try {
+      final h = _ref.read(audioHandlerProvider);
+      // A session already started before restore ran (deep link, etc.)
+      // — don't stomp it.
+      if (h.mediaItem.value != null) return;
+      final url = await _ref.read(serverUrlProvider.future);
+      final token =
+          await _ref.read(secureStorageProvider).read(key: 'session_token');
+      // Streaming needs auth + base URL; nothing to resume without them.
+      if (url == null || url.isEmpty || token == null || token.isEmpty) {
+        return;
+      }
+      final db = _ref.read(appDbProvider);
+      final row = await db.select(db.cachedResumeState).getSingleOrNull();
+      if (row == null) return;
+      final m = jsonDecode(row.json) as Map<String, dynamic>;
+      final rawTracks = (m['tracks'] as List?) ?? const [];
+      final tracks = rawTracks
+          .map((e) => TrackRef.fromJson(e as Map<String, dynamic>))
+          .toList();
+      if (tracks.isEmpty) return;
+      final idx = (m['index'] as num?)?.toInt() ?? 0;
+      final posMs = (m['position_ms'] as num?)?.toInt() ?? 0;
+      final source = m['source'] as String?;
+      await _ref.read(playerActionsProvider).restoreQueue(
+            tracks,
+            initialIndex: idx,
+            position: Duration(milliseconds: posMs),
+            source: source,
+          );
+    } catch (e, st) {
+      debugPrint('resume_controller: restore failed: $e\n$st');
+    }
+  }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    // App backgrounded / killed: persist durably right now (the debounce
+    // may not fire before teardown).
+    if (state == AppLifecycleState.paused ||
+        state == AppLifecycleState.detached) {
+      _debounce?.cancel();
+      unawaited(_persist());
+    }
+  }
+
+  void dispose() {
+    _disposed = true;
+    _debounce?.cancel();
+    WidgetsBinding.instance.removeObserver(this);
+    for (final s in _subs) {
+      s.cancel();
+    }
+    _subs.clear();
+  }
+}
+
+/// Read once at app start (app.dart postFrame). On construction it
+/// restores the last persisted session (paused) then persists
+/// queue/index/position on track change, pause, and app teardown.
+/// Disposed via ref.onDispose when the scope tears down.
+final resumeControllerProvider = Provider<ResumeController>((ref) {
+  final c = ResumeController(ref);
+  ref.onDispose(c.dispose);
+  // ignore: unawaited_futures
+  c.start();
+  return c;
+});