Merge branch 'dev' into feature/sprint1-leftovers-us02-03-04-08

origin/dev 동료(서현) PR #31 (feat(session): 면접 세션 도메인 + Core/AI 메시지 DTO) 흡수.

충돌 해결 원칙: 내 풀스택 동작 코드 유지, 동료의 좋은 부분만 흡수.

- DTO 4개 (GenerateQuestionsPayload, GenerateFollowupPayload,
  QuestionsCallbackPayload, QuestionsCallbackEnvelope): 동료 것은 미사용
  데드코드 (Service 없음). 내 풀스택 동작 코드 채택.
- InterviewSession: 내 `start/end/interrupt/cancel/updateTitleAndMemo` 유지,
  동료의 `isMaxReached()` 흡수.
- InterviewMessage: 내 `interviewer(no parent) / followup(with parent) /
  interviewee(with parent + idempotencyKey)` 시그니처 유지, 동료의
  null/blank/sequence 검증 로직을 단일 생성자에 흡수.
- InterviewMessageRepository: 내 메서드 + 동료의 `findMaxSequenceBySessionId` 흡수.
- InterviewSessionRepository, SessionContextRepository: 내 메서드 유지
  (동료 메서드는 미사용 + 필드명 부정확).
- 동료의 도메인 테스트 3개 (InterviewSessionTest, InterviewMessageTest,
  QuestionsCallbackPayloadTest): 시그니처·스키마가 내 코드와 호환 불가,
  내 SessionServiceTest/InterviewMessageServiceTest/QuestionsCallbackServiceTest
  가 같은 영역을 서비스 레이어에서 커버하므로 제거.

Author: i3months

backend/src/main/java/com/stackup/stackup/session/domain/InterviewMessage.java

@@ -67,27 +67,40 @@ public class InterviewMessage extends BaseTimeEntity {
 
     private InterviewMessage(InterviewSession session, Integer sequenceNumber, MessageRole role,
                              String content, InterviewMessage parentMessage,
-                             String idempotencyKey) {
+                             MessageStatus initialStatus, String idempotencyKey) {
+        if (session == null) {
+            throw new IllegalArgumentException("session must not be null");
+        }
+        if (content == null || content.isBlank()) {
+            throw new IllegalArgumentException("content must not be null or blank");
+        }
+        if (sequenceNumber == null || sequenceNumber < 1) {
+            throw new IllegalArgumentException("sequence must be >= 1");
+        }
         this.session = session;
         this.sequenceNumber = sequenceNumber;
         this.role = role;
         this.content = content;
         this.parentMessage = parentMessage;
+        this.status = initialStatus;
         this.idempotencyKey = idempotencyKey;
     }
 
     public static InterviewMessage interviewer(InterviewSession session, int seq, String content) {
-        return new InterviewMessage(session, seq, MessageRole.INTERVIEWER, content, null, null);
+        return new InterviewMessage(session, seq, MessageRole.INTERVIEWER, content, null,
+            MessageStatus.CREATED, null);
     }
 
     public static InterviewMessage followup(InterviewSession session, int seq, String content,
                                             InterviewMessage parent) {
-        return new InterviewMessage(session, seq, MessageRole.INTERVIEWER, content, parent, null);
+        return new InterviewMessage(session, seq, MessageRole.INTERVIEWER, content, parent,
+            MessageStatus.CREATED, null);
     }
 
     public static InterviewMessage interviewee(InterviewSession session, int seq, String content,
                                                InterviewMessage parent, String idempotencyKey) {
-        return new InterviewMessage(session, seq, MessageRole.INTERVIEWEE, content, parent, idempotencyKey);
+        return new InterviewMessage(session, seq, MessageRole.INTERVIEWEE, content, parent,
+            MessageStatus.COMPLETED, idempotencyKey);
     }
 
     public void markStatus(MessageStatus newStatus) {

backend/src/main/java/com/stackup/stackup/session/domain/InterviewMessageRepository.java

@@ -3,6 +3,8 @@
 import java.util.List;
 import java.util.Optional;
 import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.repository.query.Param;
 
 public interface InterviewMessageRepository extends JpaRepository<InterviewMessage, Long> {
 
@@ -13,4 +15,7 @@ public interface InterviewMessageRepository extends JpaRepository<InterviewMessa
     long countBySession_Id(Long sessionId);
 
     Optional<InterviewMessage> findBySession_IdAndIdempotencyKey(Long sessionId, String idempotencyKey);
+
+    @Query("select coalesce(max(m.sequenceNumber), 0) from InterviewMessage m where m.session.id = :sessionId")
+    int findMaxSequenceBySessionId(@Param("sessionId") Long sessionId);
 }

backend/src/main/java/com/stackup/stackup/session/domain/InterviewSession.java

@@ -144,6 +144,12 @@ public void incrementQuestionCount() {
         totalQuestionCount++;
     }
 
+    public boolean isMaxReached() {
+        return totalQuestionCount != null
+                && maxQuestions != null
+                && totalQuestionCount >= maxQuestions;
+    }
+
     public void updateTitleAndMemo(String title, String memo) {
         if (title != null && !title.isBlank()) {
             this.title = title;