Signed over the item, the action and the expiry together. Signing any subset would let a link be replayed against a different item or upgraded to a different action, and leaving the expiry out of the payload would let anyone holding an expired link extend it by editing the timestamp in the URL. Each of those is a test.
Compared through a second digest rather than directly, because timingSafeEqual throws when the buffers differ in length, and a truncated link is an ordinary thing to receive rather than an exception. Same idiom as the admin gate.
actionUrl returns null rather than throwing when there is no secret or no PUBLIC_URL. An unconfigured environment still sends the notification with its review link — being told an item arrived matters far more than the shortcuts do — and a link that could not be verified must never be offered in the first place.
Uses the shared trimTrailingSlashes rather than a trailing-slash regex, which is what utils.ts exports it for.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
