[privacy] remove Community Chatwoot Hub egress

Remove Community Chatwoot Hub URL/push relay/sync/registration/event/changelog
egress. changelog.js becomes a local empty-feed adapter (no axios/fetch/network)
exporting the default ChangelogApi; links.js drops the Hub changelog URL.
lib/chatwoot_hub.rb removes base_url/push_notification_url/billing_base_url/
instance_config/send_push/send_push_with_response; billing_url reads only an
explicit CHATWOOT_BILLING_URL env, HTTPS-only with host and no userinfo, and
never falls back to a Hub URL. Enterprise proprietary base_url override is
preserved (spec uses singleton_class.instance_methods(false) for edition-safety).
privacy_audit uses a narrow per-file/per-rule Enterprise exception (hub-url
only) and privacy_audit_test.sh proves forbidden Enterprise runtime lines are
still detected; deployment privacy guard unchanged. Approved by independent
five-key review deleg_49d6ee2e (passed=true, blocking arrays empty).

app/javascript/shared/constants/links.js

@@ -7,5 +7,3 @@ export const REPLY_POLICY = {
     'https://business.whatsapp.com/policy#:~:text=You%20may%20reply%20to%20a,messages%20via%20approved%20Message%20Templates.',
   TIKTOK: 'https://business-api.tiktok.com/portal/docs?id=1832184236919810',
 };
-
-export const CHANGELOG_API_URL = 'https://hub.2.chatwoot.com/changelogs';