feat: support message replies and reactions

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

@@ -2,6 +2,7 @@ package com.openisle.dto;
 
 import lombok.Data;
 import java.time.LocalDateTime;
+import java.util.List;
 
 @Data
 public class MessageDto {
@@ -10,4 +11,6 @@ public class MessageDto {
     private UserSummaryDto sender;
     private Long conversationId;
     private LocalDateTime createdAt;
+    private MessageDto replyTo;
+    private List<ReactionDto> reactions;
 }