feat: SKU新增showStock字段 - 控制是否显示库存

- ProductGoodsSkuEntity: 新增showStock(0:否 1:是)
- 价格/库存均为可空: null=线下议价/无限量

src/main/java/com/cool/modules/product/entity/ProductGoodsSkuEntity.java

@@ -26,6 +26,9 @@ public class ProductGoodsSkuEntity extends TenantEntity<ProductGoodsSkuEntity> {
     @ColumnDefine(comment = "库存")
     private Integer stock;
 
+    @ColumnDefine(comment = "是否显示库存 0:否 1:是", defaultValue = "0")
+    private Integer showStock;
+
     @ColumnDefine(comment = "规格图")
     private String image;