Schedule an HTTPS callback to fire once (`at` / `delaySeconds`) or on a bounded recurring cadence (`everySeconds` + `maxRuns`). Deliveries are signed (HMAC over timestamp + body using the returned `signingSecret`) and retried with backoff. Priced per fire ($0.002): one-shot = $0.002, recurring = $0.002 x maxRuns (prepaid; cancelling early forfeits unused fires).