Changes to POST `/api/v0/main-frontend/credit-registration-admin/account-linking/resend` - Sets off another account-linking mail for one person on one course. The mail goes to the addresses the study registry holds, and the recipient still has to open the link while signed in, so the ownership proof is intact. An override does not ask a cap for an exemption: it retires the ledger rows the cap is counting, as its own audited action, then runs the ordinary path.