feat: 이메일 인증 실패시 반환값 변경

backend/src/main/java/com/edufocus/edufocus/mail/controller/MailController.java

@@ -31,7 +31,7 @@ public class MailController {
     @GetMapping("/verify")
     public ResponseEntity<?> verifyCode(@RequestParam String code, @RequestParam String email) {
         if (!mailService.verifyCode(code, email)) {
-            return new ResponseEntity<>(HttpStatus.BAD_REQUEST);
+            return new ResponseEntity<>(HttpStatus.NOT_FOUND);
         }
         return new ResponseEntity<>(HttpStatus.OK);
     }