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/agent_status.dart

@@ -0,0 +1,38 @@
+enum AgentStatusValue { available, busy, offline }
+
+class AgentStatus {
+  final AgentStatusValue status;
+  final bool isLoggedIn;
+  final String? currentCallSid;
+  final String? lastActivity;
+  final bool availableForQueues;
+
+  AgentStatus({
+    required this.status,
+    required this.isLoggedIn,
+    this.currentCallSid,
+    this.lastActivity,
+    this.availableForQueues = true,
+  });
+
+  factory AgentStatus.fromJson(Map<String, dynamic> json) {
+    return AgentStatus(
+      status: _parseStatus(json['status'] as String),
+      isLoggedIn: json['is_logged_in'] as bool,
+      currentCallSid: json['current_call_sid'] as String?,
+      lastActivity: json['last_activity'] as String?,
+      availableForQueues: json['available_for_queues'] as bool? ?? true,
+    );
+  }
+
+  static AgentStatusValue _parseStatus(String s) {
+    switch (s) {
+      case 'available':
+        return AgentStatusValue.available;
+      case 'busy':
+        return AgentStatusValue.busy;
+      default:
+        return AgentStatusValue.offline;
+    }
+  }
+}