Cancel Checkout

Cancel an active checkout link. Sets status to 'cancelled'.

F5 security fix: requires cancel_secret in the request body. The secret is generated at link-creation time and returned to the merchant only — it is never embedded in the public checkout page. This prevents an attacker who merely knows the checkout URL from cancelling the link.

Only active links can be cancelled. Paid/expired/already-cancelled links return 409/410. Returns redirect_url so callers can redirect customers.