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/widgets/queue_card.dart

@@ -0,0 +1,37 @@
+import 'package:flutter/material.dart';
+import '../models/queue_state.dart';
+
+class QueueCard extends StatelessWidget {
+  final QueueInfo queue;
+
+  const QueueCard({super.key, required this.queue});
+
+  @override
+  Widget build(BuildContext context) {
+    return Card(
+      child: ListTile(
+        leading: CircleAvatar(
+          backgroundColor: queue.waitingCount > 0
+              ? Colors.orange.shade100
+              : Colors.green.shade100,
+          child: Text(
+            '${queue.waitingCount}',
+            style: TextStyle(
+              color: queue.waitingCount > 0 ? Colors.orange : Colors.green,
+              fontWeight: FontWeight.bold,
+            ),
+          ),
+        ),
+        title: Text(queue.name),
+        subtitle: Text(
+          queue.waitingCount > 0
+              ? '${queue.waitingCount} waiting'
+              : 'No calls waiting',
+        ),
+        trailing: queue.extension != null
+            ? Chip(label: Text('Ext ${queue.extension}'))
+            : null,
+      ),
+    );
+  }
+}