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/sse_service.dart

@@ -0,0 +1,119 @@
+import 'dart:async';
+import 'dart:convert';
+import 'dart:math';
+import 'package:dio/dio.dart';
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+import '../config/app_config.dart';
+import 'api_client.dart';
+
+class SseEvent {
+  final String event;
+  final Map<String, dynamic> data;
+
+  SseEvent({required this.event, required this.data});
+}
+
+class SseService {
+  final ApiClient _api;
+  final FlutterSecureStorage _storage = const FlutterSecureStorage();
+  final StreamController<SseEvent> _eventController =
+      StreamController<SseEvent>.broadcast();
+  final StreamController<bool> _connectionController =
+      StreamController<bool>.broadcast();
+
+  CancelToken? _cancelToken;
+  Timer? _reconnectTimer;
+  int _reconnectAttempt = 0;
+  bool _shouldReconnect = true;
+
+  Stream<SseEvent> get events => _eventController.stream;
+  Stream<bool> get connectionState => _connectionController.stream;
+
+  SseService(this._api);
+
+  Future<void> connect() async {
+    _shouldReconnect = true;
+    _reconnectAttempt = 0;
+    await _doConnect();
+  }
+
+  Future<void> _doConnect() async {
+    _cancelToken?.cancel();
+    _cancelToken = CancelToken();
+
+    try {
+      final token = await _storage.read(key: 'access_token');
+      final response = await _api.dio.get(
+        '/stream/events',
+        options: Options(
+          headers: {'Authorization': 'Bearer $token'},
+          responseType: ResponseType.stream,
+        ),
+        cancelToken: _cancelToken,
+      );
+
+      _connectionController.add(true);
+      _reconnectAttempt = 0;
+
+      final stream = response.data.stream as Stream<List<int>>;
+      String buffer = '';
+
+      await for (final chunk in stream) {
+        buffer += utf8.decode(chunk);
+        final lines = buffer.split('\n');
+        buffer = lines.removeLast(); // keep incomplete line in buffer
+
+        String? eventName;
+        String? dataStr;
+
+        for (final line in lines) {
+          if (line.startsWith('event:')) {
+            eventName = line.substring(6).trim();
+          } else if (line.startsWith('data:')) {
+            dataStr = line.substring(5).trim();
+          } else if (line.isEmpty && eventName != null && dataStr != null) {
+            try {
+              final data = jsonDecode(dataStr) as Map<String, dynamic>;
+              _eventController.add(SseEvent(event: eventName, data: data));
+            } catch (_) {}
+            eventName = null;
+            dataStr = null;
+          }
+        }
+      }
+    } catch (e) {
+      if (e is DioException && e.type == DioExceptionType.cancel) return;
+      _connectionController.add(false);
+    }
+
+    if (_shouldReconnect) {
+      _scheduleReconnect();
+    }
+  }
+
+  void _scheduleReconnect() {
+    _reconnectTimer?.cancel();
+    final delay = Duration(
+      milliseconds: min(
+        AppConfig.sseMaxReconnect.inMilliseconds,
+        AppConfig.sseReconnectBase.inMilliseconds *
+            pow(2, _reconnectAttempt).toInt(),
+      ),
+    );
+    _reconnectAttempt++;
+    _reconnectTimer = Timer(delay, _doConnect);
+  }
+
+  void disconnect() {
+    _shouldReconnect = false;
+    _reconnectTimer?.cancel();
+    _cancelToken?.cancel();
+    _connectionController.add(false);
+  }
+
+  void dispose() {
+    disconnect();
+    _eventController.close();
+    _connectionController.close();
+  }
+}