Created shared_constants.py module that loads constants from the
shared JSON file. Updated availability.py and booking.py to import
from this module instead of hardcoding values.
This ensures backend and frontend stay in sync with the same source
of truth for booking configuration.
Add check to both user and admin cancel endpoints to reject
cancellation of appointments whose slot_start is in the past.
This matches the spec requirement that cancellations can only
happen 'before the appointment'.
Added tests for both user and admin cancel endpoints.
Also includes frontend styling updates.
