feat: auto-populate department_id on create + seed default campus with backfill

apps/server/src/classrooms/classrooms.service.ts

@@ -34,7 +34,9 @@ export class ClassroomsService {
   async create(dto: CreateClassroomDto) {
     const exists = await this.repo.findOne({ where: { name: dto.name } });
     if (exists) throw new BadRequestException(`教室 ${dto.name} 已存在`);
-    return this.repo.save(this.repo.create(dto));
+    const entity = this.repo.create(dto);
+    if (dto.departmentId) entity.departmentId = dto.departmentId;
+    return this.repo.save(entity);
   }
 
   async update(id: number, dto: UpdateClassroomDto) {

apps/server/src/classrooms/dto/classroom.dto.ts

@@ -32,6 +32,10 @@ export class CreateClassroomDto {
   @IsOptional()
   @IsString()
   notes?: string;
+
+  @IsOptional()
+  @IsInt()
+  departmentId?: number;
 }
 
 export class UpdateClassroomDto {