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/voice_service.dart

@@ -0,0 +1,85 @@
+import 'dart:async';
+import 'package:twilio_voice/twilio_voice.dart';
+import 'api_client.dart';
+
+class VoiceService {
+  final ApiClient _api;
+  Timer? _tokenRefreshTimer;
+  String? _identity;
+
+  final StreamController<CallEvent> _callEventController =
+      StreamController<CallEvent>.broadcast();
+  Stream<CallEvent> get callEvents => _callEventController.stream;
+
+  VoiceService(this._api);
+
+  Future<void> initialize() async {
+    await _fetchAndRegisterToken();
+
+    TwilioVoice.instance.callEventsListener.listen((event) {
+      _callEventController.add(event);
+    });
+
+    // Refresh token every 50 minutes
+    _tokenRefreshTimer?.cancel();
+    _tokenRefreshTimer = Timer.periodic(
+      const Duration(minutes: 50),
+      (_) => _fetchAndRegisterToken(),
+    );
+  }
+
+  Future<void> _fetchAndRegisterToken() async {
+    try {
+      final response = await _api.dio.get('/voice/token');
+      final data = response.data;
+      final token = data['token'] as String;
+      _identity = data['identity'] as String;
+      await TwilioVoice.instance.setTokens(accessToken: token);
+    } catch (e) {
+      // Token fetch failed - will retry on next interval
+    }
+  }
+
+  String? get identity => _identity;
+
+  Future<void> answer() async {
+    await TwilioVoice.instance.call.answer();
+  }
+
+  Future<void> reject() async {
+    await TwilioVoice.instance.call.hangUp();
+  }
+
+  Future<void> hangUp() async {
+    await TwilioVoice.instance.call.hangUp();
+  }
+
+  Future<void> toggleMute(bool mute) async {
+    await TwilioVoice.instance.call.toggleMute(mute);
+  }
+
+  Future<void> toggleSpeaker(bool speaker) async {
+    await TwilioVoice.instance.call.toggleSpeaker(speaker);
+  }
+
+  Future<void> sendDigits(String digits) async {
+    await TwilioVoice.instance.call.sendDigits(digits);
+  }
+
+  Future<void> holdCall(String callSid) async {
+    await _api.dio.post('/calls/$callSid/hold');
+  }
+
+  Future<void> unholdCall(String callSid) async {
+    await _api.dio.post('/calls/$callSid/unhold');
+  }
+
+  Future<void> transferCall(String callSid, String target) async {
+    await _api.dio.post('/calls/$callSid/transfer', data: {'target': target});
+  }
+
+  void dispose() {
+    _tokenRefreshTimer?.cancel();
+    _callEventController.close();
+  }
+}