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/call_info.dart

@@ -0,0 +1,46 @@
+enum CallState { idle, ringing, connecting, connected, disconnected }
+
+class CallInfo {
+  final CallState state;
+  final String? callSid;
+  final String? callerNumber;
+  final Duration duration;
+  final bool isMuted;
+  final bool isSpeakerOn;
+  final bool isOnHold;
+
+  const CallInfo({
+    this.state = CallState.idle,
+    this.callSid,
+    this.callerNumber,
+    this.duration = Duration.zero,
+    this.isMuted = false,
+    this.isSpeakerOn = false,
+    this.isOnHold = false,
+  });
+
+  CallInfo copyWith({
+    CallState? state,
+    String? callSid,
+    String? callerNumber,
+    Duration? duration,
+    bool? isMuted,
+    bool? isSpeakerOn,
+    bool? isOnHold,
+  }) {
+    return CallInfo(
+      state: state ?? this.state,
+      callSid: callSid ?? this.callSid,
+      callerNumber: callerNumber ?? this.callerNumber,
+      duration: duration ?? this.duration,
+      isMuted: isMuted ?? this.isMuted,
+      isSpeakerOn: isSpeakerOn ?? this.isSpeakerOn,
+      isOnHold: isOnHold ?? this.isOnHold,
+    );
+  }
+
+  bool get isActive =>
+      state == CallState.ringing ||
+      state == CallState.connecting ||
+      state == CallState.connected;
+}