refactor: extract post dtos

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

@@ -0,0 +1,18 @@
+package com.openisle.dto;
+
+import com.openisle.model.ReactionType;
+import lombok.Data;
+
+/**
+ * DTO representing a reaction on a post or comment.
+ */
+@Data
+public class ReactionDto {
+    private Long id;
+    private ReactionType type;
+    private String user;
+    private Long postId;
+    private Long commentId;
+    private int reward;
+}
+