feat: 添加分类提案功能，包括提案表单和相关后端逻辑

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

@@ -28,4 +28,11 @@ public class PostRequest {
   // fields for poll posts
   private List<String> options;
   private Boolean multiple;
+
+  // fields for category proposal posts
+  private String proposedName;
+  private String proposedSlug;
+  private String proposalDescription;
+  private Integer approveThreshold;
+  private Integer quorum;
 }