Withdraw connection-rescue consent

The traveler's standing consent for a support agent to rebook a missed connection on their behalf. It is a consent record, not a credential: Mercury never uses it to act in the traveler's name. It has no expiry and stands until withdrawn. Withdraws the standing consent; the next connection rescue sees it gone. Idempotent: withdrawing when none stands is a no-op. Any signed-in traveler, including a Viewer, may withdraw their own consent.