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.
- Create Availability model with date, start_time, end_time
- Add availability schemas with 15-minute boundary validation
- Add admin endpoints:
- GET /api/admin/availability - query by date range
- PUT /api/admin/availability - set slots for a date
- POST /api/admin/availability/copy - copy to multiple days
- Add 26 tests covering permissions, CRUD, and validation