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.
- 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
