feat:【站内信】

backend/src/main/java/com/openisle/dto/ConversationDto.java

@@ -0,0 +1,17 @@
+package com.openisle.dto;
+
+import lombok.Getter;
+import lombok.Setter;
+
+import java.time.LocalDateTime;
+import java.util.List;
+
+@Getter
+@Setter
+public class ConversationDto {
+    private Long id;
+    private MessageDto lastMessage;
+    private List<UserSummaryDto> participants;
+    private LocalDateTime createdAt;
+    private long unreadCount;
+}