fix: resolve 5 review findings

Critical 1: Remove broken '钉钉绑定' tab (IntegConfig) — deleted /rbac/user-ding-mappings calls
Critical 2: Fix decorator placement in CreateClassDto — @IsOptional @IsArray restored to teachers
Important 1: Restore sync.service.spec.ts from 1fa3363, strip importDingTalkUsers tests
Important 2: Fix N+1 in batchImportStudents — batched find/save with In() operator
Important 3: Add migrate-student-ding-mapping.sql

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

@@ -42,12 +42,12 @@ export class CreateClassDto {
   @IsOptional() @IsArray()
   studentIds?: number[];
 
-  @IsOptional() @IsArray()
-
   @IsArray()
   @IsString({ each: true })
   @IsOptional()
   dingUserIds?: string[];
+
+  @IsOptional() @IsArray()
   teachers?: Array<{ userId: number; roleType: string; subject?: string }>;
 }