fix(backend): 库存导入用 Excel「入库日期」+ 同 key 增量更新/全同跳过

- 读取 Excel「入库日期」列(默认第12列，兼容入库日期/入库时间/入库日)，
  显式写入库存 CreatedAt，不再被 GORM 落成导入当天；新增
  parseTimeWithFallback 兼容 yyyy-MM-dd / 含时分秒 / Excel 序列号
- 同 key(商品编码|仓库，回退 名+系列+规格|仓库)：全字段一致则跳过，
  仅差异字段增量更新；库存流水仅在 quantity 实际变化时追加
- 响应新增 skipped 计数；多租户 shop_id 隔离不变

go build / vet / test 全过。

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

backend/internal/handler/import.go

@@ -496,6 +496,7 @@ func (h *ImportHandler) ImportInventory(c *gin.Context) {
 		total    int
 		imported int
 		updated  int
+		skipped  int
 		errors   []string
 	}
 	var res importResult
@@ -560,6 +561,7 @@ func (h *ImportHandler) ImportInventory(c *gin.Context) {
 	// Dynamic column detection from header row
 	colProductCode, colProductName, colSeries, colSpec, colUnit := 0, 1, 2, 3, 4
 	colQty, colPrice, colProductionDate, colBatchNo, colWarehouse, colSupplier, colRemark := 5, 6, 8, 9, 11, 13, 15
+	colStockInDate := 12
 	if len(rows) > 0 {
 		log.Printf("[import-inv] header row (%d cols): %v", len(rows[0]), rows[0])
 		for j, h := range rows[0] {
@@ -580,8 +582,10 @@ func (h *ImportHandler) ImportInventory(c *gin.Context) {
 				colPrice = j
 			case "生产日期", "生产年月":
 				colProductionDate = j
-			case "批次", "批次号":
+			case "批次", "批次号", "批次/编号/物流码":
 				colBatchNo = j
+			case "入库日期", "入库时间", "入库日":
+				colStockInDate = j
 			case "所在仓库", "仓库", "库位":
 				colWarehouse = j
 			case "供应商", "供应商名称":
@@ -620,6 +624,7 @@ func (h *ImportHandler) ImportInventory(c *gin.Context) {
 		warehouseName := cell(row, colWarehouse)
 		supplierName := cell(row, colSupplier)
 		remark := cell(row, colRemark)
+		stockInDateStr := cell(row, colStockInDate)
 
 		qty, _ := strconv.ParseFloat(qtyStr, 64)
 		if qty <= 0 {
@@ -662,6 +667,10 @@ func (h *ImportHandler) ImportInventory(c *gin.Context) {
 			productionDate = &d
 		}
 
+		// 解析入库日期：用 Excel 里的值作为库存记录的入库时间(CreatedAt)，
+		// 解析失败或为空时回退到当前时间。
+		stockInTime, _ := parseTimeWithFallback(stockInDateStr, time.Now())
+
 		whIDPtr := findWarehouse(warehouseName)
 
 		var unitPricePtr *float64
@@ -677,35 +686,93 @@ func (h *ImportHandler) ImportInventory(c *gin.Context) {
 		existing := lookupInv(productCode, prod.Name, prod.Series, prod.Spec, whIDVal)
 
 		if existing != nil {
-			updates := map[string]interface{}{
-				"quantity":       qty,
-				"product_name":   prod.Name,
-				"series":         prod.Series,
-				"spec":           prod.Spec,
-				"unit":           prod.Unit,
-				"warehouse_name": warehouseName,
-				"supplier_name":  supplierName,
-				"remark":         remark,
-				"deleted_at":     nil,
+			// 去重/增量更新：逐字段对比 Excel 行与库内现有记录，
+			// 仅把「不一致」的字段放进 updates；全部一致则跳过不写库。
+			updates := map[string]interface{}{}
+
+			if existing.Quantity != qty {
+				updates["quantity"] = qty
 			}
-			if unitPricePtr != nil {
-				updates["unit_price"] = *unitPricePtr
+			if existing.ProductName != prod.Name {
+				updates["product_name"] = prod.Name
 			}
-			if productionDate != nil {
-				updates["production_date"] = productionDate
+			if existing.Series != prod.Series {
+				updates["series"] = prod.Series
 			}
-			if batchNo != "" {
+			if existing.Spec != prod.Spec {
+				updates["spec"] = prod.Spec
+			}
+			if existing.Unit != prod.Unit {
+				updates["unit"] = prod.Unit
+			}
+			if existing.WarehouseName != warehouseName {
+				updates["warehouse_name"] = warehouseName
+			}
+			if existing.SupplierName != supplierName {
+				updates["supplier_name"] = supplierName
+			}
+			if existing.Remark != remark {
+				updates["remark"] = remark
+			}
+			if existing.BatchNo != batchNo {
 				updates["batch_no"] = batchNo
 			}
+			// 单价：仅当 Excel 提供了非零单价且与现有不同才更新
+			if unitPricePtr != nil {
+				if existing.UnitPrice == nil || *existing.UnitPrice != *unitPricePtr {
+					updates["unit_price"] = *unitPricePtr
+				}
+			}
+			// 生产日期：仅当 Excel 提供了值且与现有不同才更新
+			if productionDate != nil {
+				if existing.ProductionDate == nil ||
+					!existing.ProductionDate.Time.Equal(productionDate.Time) {
+					updates["production_date"] = productionDate
+				}
+			}
+			// 入库时间(created_at)：与现有不同才更新
+			if !existing.CreatedAt.Equal(stockInTime) {
+				updates["created_at"] = stockInTime
+			}
+
+			if len(updates) == 0 {
+				// 所有字段都一致，跳过（不写库、不写流水）
+				res.skipped++
+				continue
+			}
+
+			qtyBefore := existing.Quantity
 			if err := h.db.Model(existing).Updates(updates).Error; err != nil {
 				res.errors = append(res.errors, fmt.Sprintf("行%d: 库存更新失败: %s", i+2, err.Error()))
 				continue
 			}
-			logsToCreate = append(logsToCreate, model.InventoryLog{
-				ShopID: shopID, WarehouseID: whIDVal, ProductID: prod.ID,
-				Direction: "in", Quantity: qty, QtyBefore: existing.Quantity, QtyAfter: qty,
-				RefType: "import", RefID: 0,
-			})
+			// 仅在库存数量变化时才记一笔流水（用更新前的数量作为 QtyBefore）
+			if _, ok := updates["quantity"]; ok {
+				logsToCreate = append(logsToCreate, model.InventoryLog{
+					ShopID: shopID, WarehouseID: whIDVal, ProductID: prod.ID,
+					Direction: "in", Quantity: qty, QtyBefore: qtyBefore, QtyAfter: qty,
+					RefType: "import", RefID: 0,
+				})
+			}
+			// 同步缓存，避免同文件后续行重复比对到旧值
+			existing.Quantity = qty
+			existing.ProductName = prod.Name
+			existing.Series = prod.Series
+			existing.Spec = prod.Spec
+			existing.Unit = prod.Unit
+			existing.WarehouseName = warehouseName
+			existing.SupplierName = supplierName
+			existing.Remark = remark
+			existing.BatchNo = batchNo
+			if _, ok := updates["unit_price"]; ok {
+				existing.UnitPrice = unitPricePtr
+			}
+			if _, ok := updates["production_date"]; ok {
+				existing.ProductionDate = productionDate
+			}
+			if _, ok := updates["created_at"]; ok {
+				existing.CreatedAt = stockInTime
+			}
 			res.updated++
 		} else {
 			productIDCopy := prod.ID
@@ -726,6 +793,7 @@ func (h *ImportHandler) ImportInventory(c *gin.Context) {
 				BatchNo:        batchNo,
 				SupplierName:   supplierName,
 				Remark:         remark,
+				CreatedAt:      stockInTime, // 用 Excel「入库日期」作为入库时间
 			}
 			if err := h.db.Create(&inv).Error; err != nil {
 				res.errors = append(res.errors, fmt.Sprintf("行%d: 库存写入失败: %s", i+2, err.Error()))
@@ -756,13 +824,14 @@ func (h *ImportHandler) ImportInventory(c *gin.Context) {
 			fmt.Sprintf("未解析到任何有效行，可能列格式不匹配。识别到的表头：%s", detectedHeader))
 	}
 
-	log.Printf("[import-inv] RESULT: total=%d imported=%d updated=%d errors=%d",
-		res.total, res.imported, res.updated, len(res.errors))
+	log.Printf("[import-inv] RESULT: total=%d imported=%d updated=%d skipped=%d errors=%d",
+		res.total, res.imported, res.updated, res.skipped, len(res.errors))
 
 	c.JSON(http.StatusOK, gin.H{
 		"total":    res.total,
 		"imported": res.imported,
 		"updated":  res.updated,
+		"skipped":  res.skipped,
 		"errors":   res.errors,
 	})
 }
@@ -922,6 +991,38 @@ func parseDate(s string) model.Date {
 	return model.Date{Time: t}
 }
 
+// parseTimeWithFallback 解析「入库日期」等列的时间值。
+// 兼容：yyyy-MM-dd、yyyy-MM-dd HH:mm:ss、yyyy/MM/dd、Excel 序列号。
+// 解析失败或为空时返回 fallback（通常为 time.Now()），ok=false。
+func parseTimeWithFallback(s string, fallback time.Time) (t time.Time, ok bool) {
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return fallback, false
+	}
+	layouts := []string{
+		"2006-01-02 15:04:05",
+		"2006-01-02T15:04:05",
+		"2006-01-02 15:04",
+		"2006-01-02",
+		"2006/01/02 15:04:05",
+		"2006/01/02",
+	}
+	for _, layout := range layouts {
+		if parsed, err := time.ParseInLocation(layout, s, time.Local); err == nil {
+			return parsed, true
+		}
+	}
+	// Excel 序列号（自 1899-12-30 起的天数，可带小数表示时间）
+	if serial, err := strconv.ParseFloat(s, 64); err == nil && serial > 0 && serial < 100000 {
+		base := time.Date(1899, 12, 30, 0, 0, 0, 0, time.Local)
+		days := int(serial)
+		frac := serial - float64(days)
+		parsed := base.AddDate(0, 0, days).Add(time.Duration(frac * 24 * float64(time.Hour)))
+		return parsed, true
+	}
+	return fallback, false
+}
+
 func parsePartnerType(raw string) string {
 	hasCust := strings.Contains(raw, "客户")
 	hasSupp := strings.Contains(raw, "供应商")