- ClassroomRentalsService.create/update now upsert a ClassSchedule row with schedule_type='RENTAL' and rental_id set.
- ClassroomRentalsService.remove deletes the synced schedule row; update to 'cancelled' also removes it.
- SchedulesService.getClassroomOccupancy explicitly returns INTERNAL and RENTAL schedules.
- Make classId/teacherId/rentalId nullable in ClassSchedule entity to support rental schedules.
- Add unit tests for rental schedule sync and mixed-type occupancy.
