Queues an immediate re-delivery of ONE of the caller's own webhook deliveries. Only records with a terminal failure qualify (status ERROR or RETRY); PENDING/SUCCESS are rejected with 400 FAILED_PRECONDITION. Rate limit: at most one manual retry per record per 60 seconds (otherwise 429 RESOURCE_EXHAUSTED). Another merchant's callback_id — 403; unknown — 404. The rewritten row is returned with status PENDING, cleared response fields, and the PRESERVED retry_count (unlike the admin retry, which resets the attempt counter). Dual authentication: HMAC pair or cabinet JWT. The body is an empty JSON object.