feat(voice): bridge WhatsApp Cloud Calling to Chatwoot voice pipeline

Wires Meta's WhatsApp Cloud Calling APIs into the voice subsystem so
agents can answer / place calls on WhatsApp Cloud inboxes from the
existing Voice flow. Browser ↔ Meta WebRTC is direct (no media-server
hop); Chatwoot owns the signalling, recording upload, and lifecycle
state on the Call/Message records.

Backend
- API surface: new Api::V1::Accounts::WhatsappCallsController with
  show / accept / reject / terminate / initiate / upload_recording.
  Enforces conversation-level Pundit visibility, blocks initiate when
  the channel isn't embedded-signup voice-enabled, and surfaces the
  Meta 138006 (no-call-permission) flow as a throttled opt-in template
  send under a conversation lock so concurrent retries can't double-send.
- Whatsapp::CallService — accept/reject/terminate state machine with
  call-level locking; wraps Meta API failures (transport or business)
  as Voice::CallErrors::CallFailed so the controller renders 422.
- Whatsapp::IncomingCallService — handles Meta connect/terminate
  webhooks; pins setup:active on outbound answers, materialises a
  missed-call record when terminate arrives before connect, and
  broadcasts voice_call.* events to assignee or account streams.
- Whatsapp::Providers::WhatsappCloudService — adds pre_accept/accept/
  reject/terminate/initiate/permission-request endpoints. Uses
  configurable WHATSAPP_API_VERSION (default v22) since the Calls API
  needs v17+ and OSS phone_id_path is locked at v13.
- Audio recordings: Attachment after_create_commit enqueues
  transcription and rebroadcasts the message so the FE bubble updates
  immediately. Active Storage initializer allows audio MIME types to
  serve inline. Audio transcription drops Whisper temperature to 0 to
  suppress hallucinations on silence.
- Channel::Voice stub backs legacy inbox rows whose channel_type
  survived the DropChannelVoice migration so jbuilder .try chains
  short-circuit instead of raising.

Routes / config
- /api/v1/accounts/:id/whatsapp_calls/* (enterprise-only)
- en.yml error strings under errors.whatsapp.calls.*

The FE consumer of this API ships separately on feat/whatsapp-call-ui.

app/models/attachment.rb

@@ -104,11 +104,23 @@ class Attachment < ApplicationRecord
     audio_file_data = base_data.merge(file_metadata)
     audio_file_data.merge(
       {
+        # ActiveStorage's redirect endpoint defaults to Content-Disposition: attachment,
+        # which makes <audio> elements download instead of play. Force inline so the
+        # call-recording chip (and any other audio bubble) can stream directly.
+        data_url: inline_audio_url,
         transcribed_text: meta&.[]('transcribed_text') || ''
       }
     )
   end
 
+  def inline_audio_url
+    return '' unless file.attached?
+
+    # Proxy endpoint streams through Rails and honours `disposition: 'inline'`,
+    # unlike the redirect endpoint which always sends Content-Disposition: attachment.
+    Rails.application.routes.url_helpers.rails_storage_proxy_url(file, disposition: 'inline')
+  end
+
   def file_metadata
     metadata = {
       extension: extension,

app/models/channel/whatsapp.rb

@@ -41,8 +41,12 @@ class Channel::Whatsapp < ApplicationRecord
   end
 
   # Mirrors Channel::TwilioSms#voice_enabled? so the call subsystem can duck-type across providers.
+  # Meta's Calling API is only available via the embedded-signup whatsapp_cloud flow —
+  # 360dialog (default provider) and manual whatsapp_cloud setups can't reach the call APIs.
   def voice_enabled?
-    provider_config['calling_enabled'].present?
+    provider == 'whatsapp_cloud' &&
+      provider_config['source'] == 'embedded_signup' &&
+      provider_config['calling_enabled'].present?
   end
 
   def provider_service