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_test.go

@@ -75,6 +75,44 @@ func TestStockInHandler_FullFlow(t *testing.T) {
 	assert.Equal(t, float64(10), logs[0].Quantity)
 }
 
+// 入库明细带 sale_price 时，为该行新建的独立产品应写入 product.SalePrice（建议售价）。
+func TestStockInHandler_Create_SalePriceToProduct(t *testing.T) {
+	db := testutil.SetupTestDB()
+	shop := testutil.CreateTestShop(db, "SISALE")
+	user := testutil.CreateTestUser(db, shop.ID, "admin", "pass", "admin")
+	warehouse := testutil.CreateTestWarehouse(db, shop.ID, "Main")
+	token := getAuthToken(user.ID, shop.ID, "admin")
+	r := setupProtectedRouter(db)
+
+	w := makeRequest(r, "POST", "/api/v1/stock-in/orders", token, map[string]interface{}{
+		"warehouse_id": warehouse.ID,
+		"order_date":   time.Now().Format(time.RFC3339),
+		"items": []map[string]interface{}{
+			{
+				"product_name": "茅台飞天",
+				"series":       "53度",
+				"spec":         "500ml",
+				"quantity":     3.0,
+				"unit_price":   2680.0,
+				"sale_price":   2980.0,
+			},
+		},
+	})
+	require.Equal(t, http.StatusCreated, w.Code)
+	orderID := extractID(w)
+
+	// 找到该单据明细新建的独立产品，核对进价/售价
+	var item model.StockInItem
+	require.NoError(t, db.Where("order_id = ?", orderID).First(&item).Error)
+	require.NotZero(t, item.ProductID)
+	var prod model.Product
+	require.NoError(t, db.Where("id = ? AND shop_id = ?", item.ProductID, shop.ID).First(&prod).Error)
+	assert.Equal(t, 2680.0, prod.PurchasePrice)
+	assert.Equal(t, 2980.0, prod.SalePrice)
+	// sale_price 不落 stock_in_items（gorm:"-"），仅作产品建库入参
+	assert.Equal(t, "茅台飞天", prod.Name)
+}
+
 func TestStockInHandler_List(t *testing.T) {
 	db := testutil.SetupTestDB()
 	shop := testutil.CreateTestShop(db, "SI002")