Task 3: sync RENTAL class_schedule on rental create/update/delete

- 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.

apps/server/src/entities/class-schedule.entity.ts

@@ -21,11 +21,11 @@ export class ClassSchedule {
   @PrimaryGeneratedColumn()
   id: number;
 
-  @Column({ name: 'class_id', type: 'integer' })
-  classId: number;
+  @Column({ name: 'class_id', type: 'integer', nullable: true })
+  classId: number | null;
 
   // Forward reference — Class entity
-  @ManyToOne('Class')
+  @ManyToOne('Class', { nullable: true })
   @JoinColumn({ name: 'class_id' })
   class: unknown;
 
@@ -56,7 +56,7 @@ export class ClassSchedule {
   subject: string;
 
   @Column({ name: 'teacher_id', type: 'integer', nullable: true })
-  teacherId: number;
+  teacherId: number | null;
 
   // Forward reference — User entity
   @ManyToOne('User', { nullable: true })
@@ -67,7 +67,7 @@ export class ClassSchedule {
   scheduleType: string;
 
   @Column({ name: 'rental_id', type: 'integer', nullable: true })
-  rentalId: number;
+  rentalId: number | null;
 
   @Column({ name: 'status', length: 20, default: 'active' })
   status: string;