chore(webapp): add server-changes entry for back-office rate limit

Author: isshaddad

.server-changes/admin-back-office-rate-limit.md

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+---
+area: webapp
+type: feature
+---
+
+Add a "Back office" tab to `/admin` and a per-organization detail page at `/admin/back-office/orgs/:orgId`. The first action available on that page is editing the org's API rate limit: admins can save a `tokenBucket` override (refill rate, interval, max tokens) and see a plain-English preview of the resulting sustained rate and burst allowance. Writes are audit-logged via the server logger.