fix: fix problem with validating cron

Author: Nicklason

src/schema/dto/enqueue-repeating-schema.dto.ts

@@ -1,6 +1,8 @@
+import { IsString } from 'class-validator';
 import { IsCron } from '../../common/decorator/validation/IsCron';
 
 export class EnqueueRepeatingSchemaDto {
+  @IsString()
   @IsCron()
   readonly cron: string;
 }