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/models/queue_state.dart

@@ -0,0 +1,54 @@
+class QueueInfo {
+  final int id;
+  final String name;
+  final String type;
+  final String? extension;
+  final int waitingCount;
+
+  QueueInfo({
+    required this.id,
+    required this.name,
+    required this.type,
+    this.extension,
+    required this.waitingCount,
+  });
+
+  factory QueueInfo.fromJson(Map<String, dynamic> json) {
+    return QueueInfo(
+      id: json['id'] as int,
+      name: json['name'] as String,
+      type: json['type'] as String,
+      extension: json['extension'] as String?,
+      waitingCount: json['waiting_count'] as int,
+    );
+  }
+}
+
+class QueueCall {
+  final String callSid;
+  final String fromNumber;
+  final String toNumber;
+  final int position;
+  final String status;
+  final int waitTime;
+
+  QueueCall({
+    required this.callSid,
+    required this.fromNumber,
+    required this.toNumber,
+    required this.position,
+    required this.status,
+    required this.waitTime,
+  });
+
+  factory QueueCall.fromJson(Map<String, dynamic> json) {
+    return QueueCall(
+      callSid: json['call_sid'] as String,
+      fromNumber: json['from_number'] as String,
+      toNumber: json['to_number'] as String,
+      position: json['position'] as int,
+      status: json['status'] as String,
+      waitTime: json['wait_time'] as int,
+    );
+  }
+}