Phase 0: Add booking permissions and constants
- Add AppointmentStatus enum (booked, cancelled_by_user, cancelled_by_admin)
- Add booking permissions for regular users (book_appointment, view_own_appointments, cancel_own_appointment)
- Add availability/appointments permissions for admin (manage_availability, view_all_appointments, cancel_any_appointment)
- Add booking constants to shared/constants.json (slotDurationMinutes, maxAdvanceDays, minAdvanceDays, noteMaxLength)
- Update validate_constants.py to validate new sections