fix(backend): 商品编码改用最大序号生成不复用软删号 + 加 (shop_id,code) 唯一约束，根治重复编码

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YZ4DskSRKsSiheQonFtQvx

backend/internal/handler/product.go

@@ -62,6 +62,29 @@ func (h *ProductHandler) List(c *gin.Context) {
 	})
 }
 
+// nextProductCode 生成该门店下一个自动商品编码（P001、P002…）。
+// 取现有 P 开头编码的最大数字序号 +1，**含软删行**（deleted_at 非空也计入，不复用已删商品占用过的号）。
+// 不加行锁——并发下两请求可能算出同号，由 uk_shop_code 唯一约束 + 调用方 ErrDuplicatedKey 重试兜底。
+// 用 GORM 表达式（非 MySQL 方言 SQL），sqlite 单测也能跑。
+func nextProductCode(tx *gorm.DB, shopID uint64) (string, error) {
+	var codes []string
+	if err := tx.Model(&model.Product{}).
+		Where("shop_id = ? AND code LIKE 'P%'", shopID).
+		Pluck("code", &codes).Error; err != nil {
+		return "", err
+	}
+	maxN := 0
+	for _, c := range codes {
+		if len(c) < 2 {
+			continue
+		}
+		if n, err := strconv.Atoi(c[1:]); err == nil && n > maxN {
+			maxN = n
+		}
+	}
+	return fmt.Sprintf("P%03d", maxN+1), nil
+}
+
 // Create POST /api/v1/products
 func (h *ProductHandler) Create(c *gin.Context) {
 	shopID := middleware.GetShopID(c)
@@ -74,31 +97,24 @@ func (h *ProductHandler) Create(c *gin.Context) {
 	product.PublicID = uuid.New().String()
 	product.NamePinyin, product.NameInitials = util.ToPinyin(product.Name)
 
-	// Auto-generate product code if not provided (e.g. P001, P002)
-	// Retry up to 5 times on duplicate key to handle concurrent creates
-	if product.Code == "" {
-		var count int64
-		h.db.Model(&model.Product{}).
-			Where("shop_id = ? AND deleted_at IS NULL", shopID).
-			Count(&count)
-		product.Code = fmt.Sprintf("P%03d", count+1)
-	}
-
+	// 未显式指定编码时自动生成（事务内 max+1）；撞 uk_shop_code 唯一约束则重算下一号重试（应对并发）。
+	autoCode := product.Code == ""
 	var createErr error
 	for attempt := 0; attempt < 5; attempt++ {
-		if createErr = h.db.Create(&product).Error; createErr == nil {
+		createErr = h.db.Transaction(func(tx *gorm.DB) error {
+			if autoCode {
+				code, err := nextProductCode(tx, shopID)
+				if err != nil {
+					return err
+				}
+				product.Code = code
+			}
+			product.ID = 0
+			return tx.Create(&product).Error
+		})
+		if createErr == nil || !autoCode || !errors.Is(createErr, gorm.ErrDuplicatedKey) {
 			break
 		}
-		if !errors.Is(createErr, gorm.ErrDuplicatedKey) {
-			break
-		}
-		// Duplicate code: try next slot
-		var count int64
-		h.db.Model(&model.Product{}).
-			Where("shop_id = ? AND deleted_at IS NULL", shopID).
-			Count(&count)
-		product.ID = 0
-		product.Code = fmt.Sprintf("P%03d", count+int64(attempt)+2)
 	}
 	if createErr != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": createErr.Error()})
@@ -256,25 +272,37 @@ func (h *ProductHandler) FindOrCreate(c *gin.Context) {
 		return
 	}
 
-	var count int64
-	h.db.Model(&model.Product{}).Where("shop_id = ? AND deleted_at IS NULL", shopID).Count(&count)
 	namePinyin, nameInitials := util.ToPinyin(req.Name)
-	product = model.Product{
-		TenantBase:       model.TenantBase{ShopID: shopID},
-		PublicID:         uuid.New().String(),
-		Name:             req.Name,
-		Series:           req.Series,
-		Spec:             req.Spec,
-		Code:             fmt.Sprintf("P%03d", count+1),
-		NamePinyin:       namePinyin,
-		NameInitials:     nameInitials,
-		OriginID:         req.OriginID,
-		ShelfLifeID:      req.ShelfLifeID,
-		StorageID:        req.StorageID,
-		DescriptionDocID: req.DescriptionDocID,
+	// 事务内 max+1 生成编码；撞 uk_shop_code 唯一约束则重算下一号重试（应对并发）。
+	var createErr error
+	for attempt := 0; attempt < 5; attempt++ {
+		createErr = h.db.Transaction(func(tx *gorm.DB) error {
+			code, err := nextProductCode(tx, shopID)
+			if err != nil {
+				return err
+			}
+			product = model.Product{
+				TenantBase:       model.TenantBase{ShopID: shopID},
+				PublicID:         uuid.New().String(),
+				Name:             req.Name,
+				Series:           req.Series,
+				Spec:             req.Spec,
+				Code:             code,
+				NamePinyin:       namePinyin,
+				NameInitials:     nameInitials,
+				OriginID:         req.OriginID,
+				ShelfLifeID:      req.ShelfLifeID,
+				StorageID:        req.StorageID,
+				DescriptionDocID: req.DescriptionDocID,
+			}
+			return tx.Create(&product).Error
+		})
+		if createErr == nil || !errors.Is(createErr, gorm.ErrDuplicatedKey) {
+			break
+		}
 	}
-	if createErr := h.db.Create(&product).Error; createErr != nil {
-		// Race condition: try to find the record created by another request
+	if createErr != nil {
+		// Race condition: 并发可能已按同 name/series/spec 建好，回查返回既有
 		if h.db.Where("shop_id = ? AND name = ? AND series = ? AND spec = ? AND deleted_at IS NULL",
 			shopID, req.Name, req.Series, req.Spec).First(&product).Error == nil {
 			util.RespondSuccess(c, product)