feat: 레포지토리에 member_id로 값을 찾을 수 있는 함수 생성

#158

backend/src/main/java/sw_css/member/domain/repository/FacultyMemberRepository.java

@@ -1,8 +1,11 @@
 package sw_css.member.domain.repository;
 
+import java.util.Optional;
 import org.springframework.data.jpa.repository.JpaRepository;
 import sw_css.member.domain.FacultyMember;
 
 public interface FacultyMemberRepository extends JpaRepository<FacultyMember, Long> {
     boolean existsByMemberId(Long memberId);
+
+    Optional<FacultyMember> findByMemberId(Long memberId);
 }

backend/src/main/java/sw_css/member/domain/repository/StudentMemberRepository.java

@@ -1,10 +1,13 @@
 package sw_css.member.domain.repository;
 
+import java.util.Optional;
 import org.springframework.data.jpa.repository.JpaRepository;
 import sw_css.member.domain.StudentMember;
 
 public interface StudentMemberRepository extends JpaRepository<StudentMember, Long> {
     boolean existsById(Number studentId);
 
     boolean existsByMemberId(Number memberId);
+
+    Optional<StudentMember> findByMemberId(Number memberId);
 }