fix: null cast for refundStatus in TypeORM

apps/server/src/deposits/deposits.service.ts

@@ -167,7 +167,7 @@ export class DepositsService {
       throw new BadRequestException('未找到待审批的退款申请');
     }
 
-    deposit.refundStatus = null;
+    deposit.refundStatus = null as unknown as string;
     deposit.refundApprovedBy = userId;
     deposit.refundApprovedAt = new Date();
     deposit.refundRejectedReason = reason;