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/user.dart

@@ -0,0 +1,22 @@
+class User {
+  final int id;
+  final String login;
+  final String displayName;
+  final String? email;
+
+  User({
+    required this.id,
+    required this.login,
+    required this.displayName,
+    this.email,
+  });
+
+  factory User.fromJson(Map<String, dynamic> json) {
+    return User(
+      id: json['user_id'] as int,
+      login: json['user_login'] as String,
+      displayName: json['display_name'] as String,
+      email: json['email'] as String?,
+    );
+  }
+}