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/.metadata

@@ -0,0 +1,45 @@
+# This file tracks properties of this Flutter project.
+# Used by Flutter tool to assess capabilities and perform upgrades etc.
+#
+# This file should be version controlled and should not be manually edited.
+
+version:
+  revision: "ff37bef603469fb030f2b72995ab929ccfc227f0"
+  channel: "stable"
+
+project_type: app
+
+# Tracks metadata for the flutter migrate command
+migration:
+  platforms:
+    - platform: root
+      create_revision: ff37bef603469fb030f2b72995ab929ccfc227f0
+      base_revision: ff37bef603469fb030f2b72995ab929ccfc227f0
+    - platform: android
+      create_revision: ff37bef603469fb030f2b72995ab929ccfc227f0
+      base_revision: ff37bef603469fb030f2b72995ab929ccfc227f0
+    - platform: ios
+      create_revision: ff37bef603469fb030f2b72995ab929ccfc227f0
+      base_revision: ff37bef603469fb030f2b72995ab929ccfc227f0
+    - platform: linux
+      create_revision: ff37bef603469fb030f2b72995ab929ccfc227f0
+      base_revision: ff37bef603469fb030f2b72995ab929ccfc227f0
+    - platform: macos
+      create_revision: ff37bef603469fb030f2b72995ab929ccfc227f0
+      base_revision: ff37bef603469fb030f2b72995ab929ccfc227f0
+    - platform: web
+      create_revision: ff37bef603469fb030f2b72995ab929ccfc227f0
+      base_revision: ff37bef603469fb030f2b72995ab929ccfc227f0
+    - platform: windows
+      create_revision: ff37bef603469fb030f2b72995ab929ccfc227f0
+      base_revision: ff37bef603469fb030f2b72995ab929ccfc227f0
+
+  # User provided section
+
+  # List of Local paths (relative to this file) that should be
+  # ignored by the migrate tool.
+  #
+  # Files that are not part of the templates will be ignored by default.
+  unmanaged_files:
+    - 'lib/main.dart'
+    - 'ios/Runner.xcodeproj/project.pbxproj'