fix: review blockers — entity registration, session enum, conflict date-range, @Type decorators

apps/server/src/classes/dto/class.dto.ts

@@ -1,4 +1,5 @@
 import { IsOptional, IsString, IsNotEmpty, IsInt, IsArray, IsDateString, IsEnum } from 'class-validator';
+import { Type } from 'class-transformer';
 import { ClassType, ClassStatus, TeacherRoleType } from '../../entities';
 
 export class CreateClassDto {
@@ -84,7 +85,9 @@ export class UpdateClassDto {
 }
 
 export class QueryClassDto {
-  @IsOptional() @IsInt()
+  @IsOptional()
+  @Type(() => Number)
+  @IsInt()
   departmentId?: number;
 
   @IsOptional() @IsString()