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/android/app/build.gradle

@@ -0,0 +1,47 @@
+plugins {
+    id "com.android.application"
+    id "kotlin-android"
+    id "dev.flutter.flutter-gradle-plugin"
+    id "com.google.gms.google-services"
+}
+
+android {
+    namespace = "io.cloudhosting.twp.twp_softphone"
+    compileSdk = flutter.compileSdkVersion
+    ndkVersion = flutter.ndkVersion
+
+    compileOptions {
+        sourceCompatibility = JavaVersion.VERSION_11
+        targetCompatibility = JavaVersion.VERSION_11
+    }
+
+    kotlinOptions {
+        jvmTarget = JavaVersion.VERSION_11
+    }
+
+    defaultConfig {
+        applicationId = "io.cloudhosting.twp.twp_softphone"
+        minSdkVersion 24
+        targetSdk = flutter.targetSdkVersion
+        versionCode = flutter.versionCode
+        versionName = flutter.versionName
+        multiDexEnabled true
+    }
+
+    buildTypes {
+        release {
+            signingConfig = signingConfigs.debug
+            minifyEnabled true
+            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
+        }
+    }
+}
+
+flutter {
+    source = "../.."
+}
+
+dependencies {
+    implementation platform('com.google.firebase:firebase-bom:33.0.0')
+    implementation 'com.google.firebase:firebase-messaging'
+}