Re-send a delivery that has already finished, including one that succeeded. Use it when your
endpoint returned 2xx but you lost the event afterwards. Replay is only available through the API.

The replay keeps the same eventId and payload, so an idempotency check keyed on eventId will treat
it as a duplicate. It is signed with the webhook's current secret key and gets a fresh set of 6 attempts.

Returns 422 if the delivery has not been attempted yet or the webhook is not active.