fix: add DTO validation, transactional imports, role-not-found handling, null role guard

apps/server/src/sync/dto/import-users.dto.ts

@@ -0,0 +1,30 @@
+import {
+  IsArray,
+  IsString,
+  IsNumber,
+  IsOptional,
+  ValidateNested,
+} from 'class-validator';
+import { Type } from 'class-transformer';
+
+export class ImportUserItemDto {
+  @IsString()
+  dingUserId: string;
+
+  @IsString()
+  name: string;
+
+  @IsString()
+  mobile: string;
+
+  @IsOptional()
+  @IsNumber()
+  roleId: number | null;
+}
+
+export class ImportUsersDto {
+  @IsArray()
+  @ValidateNested({ each: true })
+  @Type(() => ImportUserItemDto)
+  users: ImportUserItemDto[];
+}