feat(cms): 优化栏目实体的banner字段定义

- 调整banner字段描述，去除背景图说明
- 新增background字段用于存储栏目背景图
- 保持mpBanner字段为移动端banner且不存表字段
- banner字段标记为非数据库字段以区分存储内容

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

@@ -69,9 +69,13 @@ public class CmsNavigation implements Serializable {
     @Schema(description = "菜单图标")
     private String icon;
 
-    @Schema(description = "栏目背景图/banner")
+    @Schema(description = "banner")
+    @TableField(exist = false)
     private String banner;
 
+    @Schema(description = "栏目背景图")
+    private String background;
+
     @Schema(description = "移动端banner")
     @TableField(exist = false)
     private String mpBanner;