Admin Back Office Rate Limit

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.