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/services/base/send_on_channel_service.rb

@@ -46,7 +46,8 @@ class Base::SendOnChannelService
   def invalid_message?
     # private notes aren't send to the channels
     # we should also avoid the case of message loops, when outgoing messages are created from channel
-    message.private? || outgoing_message_originated_from_channel?
+    # voice_call bubbles are call status indicators, not deliverable messages
+    message.private? || outgoing_message_originated_from_channel? || message.content_type == 'voice_call'
   end
 
   def validate_target_channel