feat(shop): 添加会员价字段

- 在ShopGoodsSku实体中新增buyingPrice字段
- 使用@Schema注解描述会员价含义
- 扩展商品SKU数据结构以支持会员价功能

src/main/java/com/gxwebsoft/shop/entity/ShopGoodsSku.java

@@ -45,6 +45,9 @@ public class ShopGoodsSku implements Serializable {
     @Schema(description = "成本价")
     private BigDecimal cost;
 
+    @Schema(description = "会员价")
+    private BigDecimal buyingPrice;
+
     @Schema(description = "库存")
     private Integer stock;