feat(player): audio focus, interruptions & becoming-noisy

The Flutter client had no audio_session integration, so it didn't pause
for phone calls / other media, didn't duck for navigation prompts, and
kept blasting the phone speaker when earbuds were unplugged.

Add audio_session ^0.2.3 and configure AudioSessionConfiguration.music()
in MinstrelAudioHandler (best-effort, fully try-caught):
- becomingNoisy -> pause (no speaker blast on unplug/BT drop)
- interruption begin: duck -> lower volume; pause/unknown -> pause,
  remembering whether we were actively playing
- interruption end: duck -> restore volume; pause -> resume only if we
  paused it and the session isn't torn down (guards the idle-teardown
  -during-long-call edge; re-init is resume-last-session territory);
  unknown -> no auto-resume

pubspec.lock regenerated by build/CI.

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

flutter_client/lib/player/audio_handler.dart

@@ -2,6 +2,7 @@ import 'dart:async';
 import 'dart:io';
 
 import 'package:audio_service/audio_service.dart';
+import 'package:audio_session/audio_session.dart';
 import 'package:flutter/foundation.dart';
 import 'package:just_audio/just_audio.dart';
 import 'package:path_provider/path_provider.dart';
@@ -38,6 +39,7 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
     // the index and the eviction loop can't reclaim them.
     _player.bufferedPositionStream
         .listen((_) => unawaited(_maybeRegisterStreamCache()));
+    unawaited(_configureAudioSession());
   }
 
   final AudioPlayer _player = AudioPlayer();
@@ -68,6 +70,15 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
   /// cancelled the moment playback resumes or a new queue is set.
   Timer? _idleStopTimer;
 
+  /// Player volume captured when an OS duck interruption begins,
+  /// restored when it ends. Null when not currently ducked.
+  double? _volumeBeforeDuck;
+
+  /// True when an interruption (call / other media) paused playback that
+  /// was actively going, so a transient (pause-type) interruption end
+  /// auto-resumes. Cleared on resume or a non-resumable end.
+  bool _interruptedWhilePlaying = false;
+
   /// True while _fillRemainingSources is doing backward-fill inserts
   /// at index 0..initialIndex-1. Each insert shifts the player's
   /// currentIndex (it tracks the actively-playing source through
@@ -729,6 +740,65 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
     }
     unawaited(stop());
   }
+
+  /// Configures the OS audio session for music playback and wires
+  /// interruption + becoming-noisy handling. just_audio auto-activates /
+  /// deactivates the session around playback once it's configured, but
+  /// does NOT handle interruptions or the headphones-unplugged event
+  /// itself — that's done here. Best effort: a failure must not break
+  /// playback.
+  Future<void> _configureAudioSession() async {
+    try {
+      final session = await AudioSession.instance;
+      await session.configure(AudioSessionConfiguration.music());
+      session.interruptionEventStream.listen(_onInterruption);
+      session.becomingNoisyEventStream.listen((_) {
+        // Headphones unplugged / Bluetooth disconnected — never blast the
+        // phone speaker; pause like every other media app.
+        unawaited(_player.pause());
+      });
+    } catch (e, st) {
+      debugPrint('audio_handler: audio session configure failed: $e\n$st');
+    }
+  }
+
+  void _onInterruption(AudioInterruptionEvent event) {
+    if (event.begin) {
+      switch (event.type) {
+        case AudioInterruptionType.duck:
+          // Transient: another app wants the foreground briefly. Lower
+          // our volume rather than stopping (OS may also auto-duck).
+          _volumeBeforeDuck = _player.volume;
+          unawaited(_player.setVolume(0.3));
+        case AudioInterruptionType.pause:
+        case AudioInterruptionType.unknown:
+          // Call / other media took focus. Remember whether we were
+          // actively playing so a transient end can resume us.
+          _interruptedWhilePlaying = _player.playing &&
+              _player.processingState != ProcessingState.completed;
+          unawaited(_player.pause());
+      }
+    } else {
+      switch (event.type) {
+        case AudioInterruptionType.duck:
+          unawaited(_player.setVolume(_volumeBeforeDuck ?? 1.0));
+          _volumeBeforeDuck = null;
+        case AudioInterruptionType.pause:
+          // Transient interruption ended — resume only if WE paused it
+          // and the session is still alive (a long call may have let the
+          // #52 idle timer tear it down; re-init is resume-last-session
+          // territory, out of scope here).
+          if (_interruptedWhilePlaying &&
+              _player.processingState != ProcessingState.idle) {
+            unawaited(_player.play());
+          }
+          _interruptedWhilePlaying = false;
+        case AudioInterruptionType.unknown:
+          // Unknown end — do not auto-resume (could surprise the user).
+          _interruptedWhilePlaying = false;
+      }
+    }
+  }
 }
 
 /// Adapter that lets the audio handler call into the app's

flutter_client/pubspec.yaml

@@ -14,6 +14,11 @@ dependencies:
   dio: ^5.7.0
   just_audio: ^0.10.5
   audio_service: ^0.18.15
+  # Audio focus + interruptions + becoming-noisy. just_audio auto-manages
+  # session activation once configured; the app must still handle
+  # interruption/becoming-noisy itself (just_audio does not). Same author
+  # as just_audio/audio_service so versions track together.
+  audio_session: ^0.2.3
   flutter_secure_storage: ^10.1.0
   go_router: ^17.2.3
   flutter_svg: ^2.0.16