[Fix] 익명 번호 결정 방식 MAX 기반으로 변경 및 V5 마이그레이션

backend/src/main/java/com/example/Piroin/project/domain/question/repository/QuestionAnonymousIdentityRepository.java

@@ -5,6 +5,8 @@
 import com.example.Piroin.project.domain.user.entity.User;
 import com.example.Piroin.project.domain.user.enums.Role;
 import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.repository.query.Param;
 
 import java.util.Optional;
 
@@ -18,4 +20,10 @@ public interface QuestionAnonymousIdentityRepository extends JpaRepository<Quest
     // 용도: 새 익명 번호 발급 시 역할별로 따로 카운트
     // MEMBER → 익명1, 익명2... / ADMIN → 운영진1, 운영진2...
     int countByQuestionAndUser_Role(Question question, Role role);
+
+    @Query("SELECT COALESCE(MAX(a.anonymousNo), 0) FROM QuestionAnonymousIdentity a " + "WHERE a.question = :question AND a.user.role = :role")
+    int findMaxAnonymousNoByQuestionAndRole(
+            @Param("question") Question question,
+            @Param("role") Role role
+    );
 }

backend/src/main/java/com/example/Piroin/project/domain/question/service/QuestionService.java

@@ -196,7 +196,7 @@ private String assignAnonymousIdentity(Question question, User commenter) {
                 .orElseGet(() -> {
                     // 처음 댓글 다는 유저 → 역할별 카운트 기반으로 새 번호 부여
                     int nextNo = anonymousIdentityRepository
-                            .countByQuestionAndUser_Role(question, commenter.getRole()) + 1;
+                            .findMaxAnonymousNoByQuestionAndRole(question, commenter.getRole()) + 1;
 
                     anonymousIdentityRepository.save(QuestionAnonymousIdentity.builder()
                             .question(question)

backend/src/main/resources/db/migration/V5__add_unique_constraint_question_anonymous_identity.sql

@@ -0,0 +1,3 @@
+ALTER TABLE question_anonymous_identity
+    ADD CONSTRAINT uq_question_anonymous_identity_question_user
+    UNIQUE (question_id, user_id);