fix: 임시저장 편지 조회 API 500 에러 수정 (WR9-123)

- JPQL 쿼리와 DTO 클래스 필드명 일치시킴 (l.id → letterId 별칭 추가)
- LetterDraftResponse 클래스에 @AllArgsConstructor 어노테이션 추가
- 매칭 여부 판단 로직 명확히 표현 (CASE WHEN 구문 별칭 추가)

Author: Kojinyoung7220

src/main/java/io/crops/warmletter/domain/letter/dto/response/LetterDraftResponse.java

@@ -10,6 +10,7 @@
 
 @Getter
 @Builder
+@AllArgsConstructor
 public class LetterDraftResponse {
 
     private final Long letterId;

src/main/java/io/crops/warmletter/domain/letter/repository/LetterRepository.java

@@ -51,10 +51,20 @@ Page<Letter> findDeliveredOrMyLettersByMatchingId(
     List<Letter> findByReceiverIdAndStatus(Long currentUserId, Status status);
 
     @Query("SELECT new io.crops.warmletter.domain.letter.dto.response.LetterDraftResponse(" +
-            "l.id, l.writerId, l.receiverId, l.parentLetterId, " +
-            "l.title, l.content, l.category, l.paperType, l.fontType, l.status, " +
-            "CASE WHEN m.id IS NOT NULL THEN m.isActive ELSE false END, " +
-            "l.deliveryStartedAt, l.deliveryCompletedAt, l.matchingId) " +
+            "l.id AS letterId, " +
+            "l.writerId, " +
+            "l.receiverId, " +
+            "l.parentLetterId, " +
+            "l.title, " +
+            "l.content, " +
+            "l.category, " +
+            "l.paperType, " +
+            "l.fontType, " +
+            "l.status, " +
+            "CASE WHEN m.id IS NOT NULL THEN m.isActive ELSE false END AS matched, " +
+            "l.deliveryStartedAt, " +
+            "l.deliveryCompletedAt, " +
+            "l.matchingId) " +
             "FROM Letter l LEFT JOIN LetterMatching m ON l.matchingId = m.id " +
             "WHERE l.writerId = :writerId AND l.status = :status")
     List<LetterDraftResponse> findDraftLettersWithMatching(@Param("writerId") Long writerId,