Returns links to the published site's cancel/reschedule pages for the appointment with the given UID. Intended for callers (e.g. a chatbot) that verify appointment ownership out-of-band and then send the customer a link to complete the action themselves, rather than cancelling/rescheduling directly. Does not check whether the UID belongs to an existing appointment; an unknown UID yields links that simply do not resolve to a booking when visited. An optional lang (published site language locale code, e.g. en, de, pt-br) picks the correctly-localized page on multilingual sites; an unknown or disabled locale returns 400.