feat(whatsapp-call): wire up media-server end-to-end

Fixes the four blockers identified in the review so the WhatsApp call
feature actually flows through the Go media-server sidecar:

1. MediaServerClient#create_session now sends `direction` — Go rejected
   every session create before this (400). Threaded through to both
   CallService#accept_via_media_server (incoming) and
   WhatsappCallsController#create_outbound_call_via_media_server (outgoing).

2. Outbound SDP response key aligned — Rails now reads `meta_sdp_offer`
   (the media-server's offer for Meta), not the old `meta_sdp_answer`
   which never existed in the Go response.

3. Media-server callbacks moved to top-level /callbacks/media_server/<event>
   with a new controller that skips Devise account scoping and is guarded
   solely by MEDIA_SERVER_AUTH_TOKEN. Matches the Go client's existing
   request path, so agent_disconnected / recording_ready / session_terminated
   actually reach Rails instead of 404'ing on the account-auth gate.

4. Inbox serializer exposes `media_server_enabled` so the dashboard picks
   the server-relay code path when the feature is enabled.

Also lands the outbound server-relay connect flow that was missing:
Go gains POST /sessions/{id}/meta-answer so Rails can feed Meta's SDP
answer back into Peer A when the contact picks up; the webhook handler
then asks the server for a Peer B offer and broadcasts it to the agent
browser instead of Meta's raw answer.

Procfile.dev runs the Go sidecar alongside Rails/sidekiq/vite in dev;
.env gets the three MEDIA_SERVER_* variables.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/views/api/v1/models/_inbox.json.jbuilder

@@ -130,6 +130,7 @@ if resource.whatsapp?
   json.provider_config resource.channel.try(:provider_config) if Current.account_user&.administrator?
   json.reauthorization_required resource.channel.try(:reauthorization_required?)
   json.calling_enabled resource.channel.try(:provider_config)&.dig('calling_enabled') || false
+  json.media_server_enabled Call.media_server_enabled?
 end
 
 ## Voice Channel Attributes