Add TWP Softphone Flutter app and complete mobile backend API

Backend: Add /voice/token endpoint with AccessToken + VoiceGrant for
mobile VoIP, implement unhold_call() with call leg detection, wire FCM
push notifications into call queue and webhook missed call handlers,
add data-only FCM message support for Android background wake, and add
Twilio API Key / Push Credential settings fields.

Flutter app: Full softphone with Twilio Voice SDK integration, JWT auth
with auto-refresh, SSE real-time queue updates, FCM push notifications,
Material 3 UI with dashboard, active call screen, dialpad, and call
controls (mute/speaker/hold/transfer).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

mobile/lib/services/push_notification_service.dart

@@ -0,0 +1,78 @@
+import 'package:firebase_core/firebase_core.dart';
+import 'package:firebase_messaging/firebase_messaging.dart';
+import 'package:flutter_local_notifications/flutter_local_notifications.dart';
+import 'api_client.dart';
+
+@pragma('vm:entry-point')
+Future<void> _firebaseMessagingBackgroundHandler(RemoteMessage message) async {
+  await Firebase.initializeApp();
+  // VoIP pushes are handled natively by twilio_voice plugin.
+  // Other data messages can show a local notification if needed.
+}
+
+class PushNotificationService {
+  final ApiClient _api;
+  final FirebaseMessaging _messaging = FirebaseMessaging.instance;
+  final FlutterLocalNotificationsPlugin _localNotifications =
+      FlutterLocalNotificationsPlugin();
+
+  PushNotificationService(this._api);
+
+  Future<void> initialize() async {
+    FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);
+
+    await _messaging.requestPermission(
+      alert: true,
+      badge: true,
+      sound: true,
+      criticalAlert: true,
+    );
+
+    // Initialize local notifications
+    const androidSettings =
+        AndroidInitializationSettings('@mipmap/ic_launcher');
+    const initSettings = InitializationSettings(android: androidSettings);
+    await _localNotifications.initialize(initSettings);
+
+    // Get and register FCM token
+    final token = await _messaging.getToken();
+    if (token != null) {
+      await _registerToken(token);
+    }
+
+    // Listen for token refresh
+    _messaging.onTokenRefresh.listen(_registerToken);
+
+    // Handle foreground messages (non-VoIP)
+    FirebaseMessaging.onMessage.listen(_handleForegroundMessage);
+  }
+
+  Future<void> _registerToken(String token) async {
+    try {
+      await _api.dio.post('/fcm/register', data: {'fcm_token': token});
+    } catch (_) {}
+  }
+
+  void _handleForegroundMessage(RemoteMessage message) {
+    final data = message.data;
+    final type = data['type'];
+
+    // VoIP incoming_call is handled by twilio_voice natively
+    if (type == 'incoming_call') return;
+
+    // Show local notification for other types (missed call, queue alert, etc.)
+    _localNotifications.show(
+      message.hashCode,
+      data['title'] ?? 'TWP Softphone',
+      data['body'] ?? '',
+      const NotificationDetails(
+        android: AndroidNotificationDetails(
+          'twp_general',
+          'General Notifications',
+          importance: Importance.high,
+          priority: Priority.high,
+        ),
+      ),
+    );
+  }
+}