feat(voice): subscribe to calls webhook + gate WhatsApp calling on channel_voice flag

- Add 'calls' to the WhatsApp WABA override_callback subscribed_fields so a
  fresh embedded-signup connection (or a re-register) automatically receives
  call webhooks from Meta. Previously required a manual App Dashboard step.
- Gate Channel::Whatsapp#voice_enabled? on the account-level 'channel_voice'
  feature flag, matching the entitlement Twilio voice channel creation
  already requires. Cascades through the controller, webhook services, and
  inbox jbuilder voice_enabled serialization (FE button hides automatically).

app/models/channel/whatsapp.rb

@@ -46,7 +46,8 @@ class Channel::Whatsapp < ApplicationRecord
   def voice_enabled?
     provider == 'whatsapp_cloud' &&
       provider_config['source'] == 'embedded_signup' &&
-      provider_config['calling_enabled'].present?
+      provider_config['calling_enabled'].present? &&
+      account.feature_enabled?('channel_voice')
   end
 
   def provider_service

app/services/whatsapp/facebook_api_client.rb

@@ -86,7 +86,7 @@ class Whatsapp::FacebookApiClient
       body: {
         override_callback_uri: callback_url,
         verify_token: verify_token,
-        subscribed_fields: %w[messages smb_message_echoes]
+        subscribed_fields: %w[messages smb_message_echoes calls]
       }.to_json
     )