feat(chat): 멤버가 최근에 대화한 캐릭터 목록

- ChatCharacterRepository.kt의 JPQL 정렬 절을 `ORDER BY MAX(COALESCE(m.createdAt, r.createdAt)) DESC`로 변경

src/main/kotlin/kr/co/vividnext/sodalive/chat/character/repository/ChatCharacterRepository.kt

@@ -59,7 +59,7 @@ interface ChatCharacterRepository : JpaRepository<ChatCharacter, Long> {
               AND pc.isActive = true
               AND r.isActive = true
             GROUP BY c.id
-            ORDER BY COALESCE(MAX(m.createdAt), r.createdAt) DESC
+            ORDER BY MAX(COALESCE(m.createdAt, r.createdAt)) DESC
         """
     )
     fun findRecentCharactersByMember(