fix(backend): 往来单位下拉返回全部 + 入库售价写入产品

- partner List 分页上限 200→1000：避免下拉请求大页被 ValidatePageSize
  回退到默认 20（超上限返回默认值而非截断），供应商/客户下拉一次取全部
- 入库明细 sale_price 建独立产品时写入 product.SalePrice（建议售价）；
  SalePrice 为 gorm:"-" 仅作建库入参，不落 stock_in_items 表
- 补 partner 大页返回全部、入库售价入产品两条回归测试

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/handler/stock_in.go

@@ -228,7 +228,7 @@ func (h *StockInHandler) Create(c *gin.Context) {
 			if it.BatchNo == "" {
 				it.BatchNo = fmt.Sprintf("%s-%02d", req.OrderNo, i+1)
 			}
-			prod, e := createIndependentProduct(tx, shopID, it.ProductName, it.Series, it.Spec, it.BatchNo, it.ProductionDate, it.UnitPrice)
+			prod, e := createIndependentProduct(tx, shopID, it.ProductName, it.Series, it.Spec, it.BatchNo, it.ProductionDate, it.UnitPrice, it.SalePrice)
 			if e != nil {
 				return e
 			}
@@ -277,7 +277,7 @@ func (h *StockInHandler) Update(c *gin.Context) {
 				it.BatchNo = fmt.Sprintf("%s-%02d", order.OrderNo, i+1)
 			}
 			// 编辑草稿:旧明细已删,每条按新模型重建独立产品（旧草稿 product 无库存,暂留待后续清理）
-			prod, e := createIndependentProduct(tx, shopID, it.ProductName, it.Series, it.Spec, it.BatchNo, it.ProductionDate, it.UnitPrice)
+			prod, e := createIndependentProduct(tx, shopID, it.ProductName, it.Series, it.Spec, it.BatchNo, it.ProductionDate, it.UnitPrice, it.SalePrice)
 			if e != nil {
 				return e
 			}