feat(cms): 添加页面附件字段支持

- 在 CmsPage 实体类中新增 attachments 字段
- attachments 字段类型为 JSON 数组字符串，包含 name/url/size/ext/sort 信息
- 为 attachments 字段添加相应的 @Schema 注解描述

src/main/java/com/gxwebsoft/cms/entity/CmsPage.java

@@ -64,6 +64,9 @@ public class CmsPage implements Serializable {
     @Schema(description = "备注")
     private String comments;
 
+    @Schema(description = "页面附件(JSON 数组字符串，元素含 name/url/size/ext/sort)")
+    private String attachments;
+
     @Schema(description = "阅读量")
     private Integer views;