Retrieve a booking

Books an appointment. The start is re-checked exactly the way the public booking page re-checks it: the staff member's working hours, the location's hours, existing bookings, classes, rooms, and the location's booking rules (same day, how far ahead, minimum notice). The visit is sized for the staff member doing it (their own duration, waiting period and cleanup time for the service). A taken or closed time returns `409 slot_unavailable`; a broken booking rule returns `422 booking_rule_violation`. Without `technician_id`, at a location where every appointment needs a staff member, the first free staff member who performs the service (in the location's booking priority order) is assigned: the same person `GET /availability` named for that slot. At a location that books onto rooms, the appointment is booked onto a room with no staff member (the service must have a room there, else `422 service_not_bookable`).