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/providers/auth_provider.dart

@@ -0,0 +1,107 @@
+import 'package:flutter/foundation.dart';
+import '../models/user.dart';
+import '../services/api_client.dart';
+import '../services/auth_service.dart';
+import '../services/voice_service.dart';
+import '../services/push_notification_service.dart';
+import '../services/sse_service.dart';
+
+enum AuthState { unauthenticated, authenticating, authenticated }
+
+class AuthProvider extends ChangeNotifier {
+  final ApiClient _apiClient;
+  late final AuthService _authService;
+  late final VoiceService _voiceService;
+  late final PushNotificationService _pushService;
+  late final SseService _sseService;
+
+  AuthState _state = AuthState.unauthenticated;
+  User? _user;
+  String? _error;
+
+  AuthState get state => _state;
+  User? get user => _user;
+  String? get error => _error;
+  VoiceService get voiceService => _voiceService;
+  SseService get sseService => _sseService;
+  ApiClient get apiClient => _apiClient;
+
+  AuthProvider(this._apiClient) {
+    _authService = AuthService(_apiClient);
+    _voiceService = VoiceService(_apiClient);
+    _pushService = PushNotificationService(_apiClient);
+    _sseService = SseService(_apiClient);
+
+    _apiClient.onForceLogout = _handleForceLogout;
+  }
+
+  Future<void> tryRestoreSession() async {
+    final restored = await _authService.tryRestoreSession();
+    if (restored) {
+      _state = AuthState.authenticated;
+      await _initializeServices();
+      notifyListeners();
+    }
+  }
+
+  Future<void> login(String serverUrl, String username, String password) async {
+    _state = AuthState.authenticating;
+    _error = null;
+    notifyListeners();
+
+    try {
+      _user = await _authService.login(serverUrl, username, password);
+      _state = AuthState.authenticated;
+      await _initializeServices();
+    } catch (e) {
+      _state = AuthState.unauthenticated;
+      _error = e.toString().replaceFirst('Exception: ', '');
+    }
+    notifyListeners();
+  }
+
+  Future<void> _initializeServices() async {
+    try {
+      await _pushService.initialize();
+    } catch (_) {}
+    try {
+      await _voiceService.initialize();
+    } catch (_) {}
+    try {
+      await _sseService.connect();
+    } catch (_) {}
+  }
+
+  Future<void> logout() async {
+    _voiceService.dispose();
+    _sseService.disconnect();
+    await _authService.logout();
+
+    _state = AuthState.unauthenticated;
+    _user = null;
+    _error = null;
+
+    // Re-create services for potential re-login
+    _voiceService = VoiceService(_apiClient);
+    _pushService = PushNotificationService(_apiClient);
+    _sseService = SseService(_apiClient);
+
+    notifyListeners();
+  }
+
+  void _handleForceLogout() {
+    _state = AuthState.unauthenticated;
+    _user = null;
+    _error = 'Session expired. Please log in again.';
+    _sseService.disconnect();
+    notifyListeners();
+  }
+
+  @override
+  void dispose() {
+    _authService.dispose();
+    _voiceService.dispose();
+    _sseService.dispose();
+    super.dispose();
+  }
+}