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/dialpad.dart

@@ -0,0 +1,55 @@
+import 'package:flutter/material.dart';
+
+class Dialpad extends StatelessWidget {
+  final void Function(String digit) onDigit;
+  final VoidCallback onClose;
+
+  const Dialpad({super.key, required this.onDigit, required this.onClose});
+
+  static const _keys = [
+    ['1', '2', '3'],
+    ['4', '5', '6'],
+    ['7', '8', '9'],
+    ['*', '0', '#'],
+  ];
+
+  @override
+  Widget build(BuildContext context) {
+    return Padding(
+      padding: const EdgeInsets.symmetric(horizontal: 48),
+      child: Column(
+        mainAxisSize: MainAxisSize.min,
+        children: [
+          ..._keys.map((row) => Row(
+                mainAxisAlignment: MainAxisAlignment.spaceEvenly,
+                children: row
+                    .map((key) => Padding(
+                          padding: const EdgeInsets.all(4),
+                          child: InkWell(
+                            onTap: () => onDigit(key),
+                            borderRadius: BorderRadius.circular(40),
+                            child: Container(
+                              width: 64,
+                              height: 64,
+                              alignment: Alignment.center,
+                              child: Text(
+                                key,
+                                style: Theme.of(context)
+                                    .textTheme
+                                    .headlineSmall,
+                              ),
+                            ),
+                          ),
+                        ))
+                    .toList(),
+              )),
+          const SizedBox(height: 8),
+          TextButton(
+            onPressed: onClose,
+            child: const Text('Close'),
+          ),
+        ],
+      ),
+    );
+  }
+}