[Task] 유효성 검사 / 백엔드

### **ISSUE**

- Group: `server`
- Type: `feature`
- Detail: 유효성 이중 검사

### **TODO**

- [ ] users controllers 유효성 검사 / 클라이언트랑 이중검사(권장)

### **Estimated time**

> ### **`3h`**

### **Labels**

- Estimated time: `E: 3h`
- Group : `server`
- Sprint: `Sprint2`
- Urgency: `Low`

Start by locating the users controllers in the server group and reviewing the existing client-side validation. Compare the two validation paths and confirm that the users controllers also perform the required checks; done means the server-side validation is present and aligned with the client-side checks.

Written by the indexing model from the issue text.