Retry a dead-lettered webhook event

Redelivers the event to its endpoint exactly once and returns the outcome
inline; this does not re-enter the automatic backoff schedule. On success the
event becomes delivered, on failure it returns to dead_letter and can be
retried again once the underlying problem is fixed. Only dead_letter events
are retryable - a pending event is already queued and a delivered one has
nothing to resend.