fix: 회원가입시 중복 아이디 불가하게 설정

- findByNickname 대신 findByLoginId 로 올바르게 로그인id를 찾음

src/main/java/com/solucitation/midpoint_backend/domain/member/repository/MemberRepository.java

@@ -10,6 +10,8 @@
 public interface MemberRepository extends JpaRepository<Member, String> {
     Optional<Member> findByEmail(String email);
     Optional<Member> findByNickname(String nickname);
+
+    Optional<Member> findByLoginId(String loginId);
     Optional<Member> findByEmailOrLoginId(String email, String nickname);
     Optional<Member> findByName(String name);
 }

src/main/java/com/solucitation/midpoint_backend/domain/member/service/MemberService.java

@@ -98,7 +98,7 @@ public boolean isNicknameAlreadyInUse(String nickname) {
      * @return 로그인 아이디가 이미 사용 중이면 true, 아니면 false
      */
     public boolean isLoginIdAlreadyInUse(String loginId) {
-        return memberRepository.findByNickname(loginId).isPresent();
+        return memberRepository.findByLoginId(loginId).isPresent();
     }
 
     /**