RFC 8058 one-click unsubscribe

What Gmail's and Outlook's own unsubscribe buttons call, from the List-Unsubscribe header the optional emails carry. Same token and same effect as the GET. The token is an HMAC over userId:family minted into the email itself — no session, nothing to enumerate, and no expiry, because an unsubscribe link in a two-year-old email must still work. Applies to the three OPTIONAL families only; essential email (sign-in codes, receipts, security alerts, terms updates) has no opt-out. Every write merges, so unsubscribing from one family never resubscribes another.