Redeliver a webhook event

Redelivers the event to its endpoint exactly once and returns the outcome
inline; this does not re-enter the automatic backoff schedule. Use it to replay
an event your endpoint acknowledged with a 2xx but failed to process, and to
retry one that exhausted its automatic attempts.

Both delivered and dead_letter events are retryable; a pending event is
still queued for automatic delivery and returns 409. Replaying an already
delivered event leaves it delivered even if the replay fails - the original
delivery still happened - so check success and error_message for the outcome
of this attempt.