refactor: extract post dtos

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

@@ -0,0 +1,21 @@
+package com.openisle.dto;
+
+import lombok.Data;
+
+import java.time.LocalDateTime;
+import java.util.List;
+
+/**
+ * DTO representing a comment and its nested replies.
+ */
+@Data
+public class CommentDto {
+    private Long id;
+    private String content;
+    private LocalDateTime createdAt;
+    private AuthorDto author;
+    private List<CommentDto> replies;
+    private List<ReactionDto> reactions;
+    private int reward;
+}
+