feat: add user medal selection and display

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

@@ -1,6 +1,7 @@
 package com.openisle.dto;
 
 import lombok.Data;
+import com.openisle.model.MedalType;
 
 /**
  * DTO representing a post or comment author.
@@ -10,5 +11,6 @@ public class AuthorDto {
     private Long id;
     private String username;
     private String avatar;
+    private MedalType displayMedal;
 }