Fixes https://github.com/chatwoot/chatwoot/issues/13097
### Problem
The PR #12176 removed the `before_save :setup_webhooks` callback to fix
a race condition where Meta's webhook verification request arrived
before the channel was saved to the database. This change broke manual
WhatsApp Cloud channel setup. While embedded signup explicitly calls
`channel.setup_webhooks` in `EmbeddedSignupService`, manual setup had no
equivalent call - meaning the `subscribed_apps` endpoint was never
invoked and Meta never sent webhook events to Chatwoot.
### Solution
Added an `after_commit` callback that triggers webhook setup for manual
WhatsApp Cloud channels
7e4d93f649