fix(backend): 架构质量改进批次二 (#32-37)

- #32 License 激活迁移到 license_devices 表：Activate/Verify/Deactivate 全部改用
  license_devices，新增 max_devices 校验和 GET /license/devices 端点；
  Activate 现在校验 shop_id 防跨租户激活
- #33 checkInventory 从 StockOutHandler 移到 StockService.CheckInventoryAvailability
- #34 新增 util/response.go 统一错误响应工具（RespondError/RespondSuccess/RespondCreated）
- #35 生产模式 CORS Origin='*' 启动时 Fatal
- #36 生产模式 License 私钥未配置启动时 Fatal
- #37 新增 util/page.go ValidatePageSize，应用到 partner/product/stock_in/stock_out handler

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/service/stock.go

@@ -287,3 +287,46 @@ func (s *StockService) GenerateOrderNo(shopID uint64, orderType string) (string,
 	})
 	return no, err
 }
+
+// CheckInventoryAvailability validates that the warehouse has sufficient stock for each item.
+// Returns an error describing the first shortage encountered.
+func (s *StockService) CheckInventoryAvailability(shopID, warehouseID uint64, items []model.StockOutItem) error {
+	if len(items) == 0 {
+		return nil
+	}
+
+	productIDs := make([]uint64, 0, len(items))
+	for _, item := range items {
+		productIDs = append(productIDs, item.ProductID)
+	}
+
+	type inventorySum struct {
+		ProductID uint64
+		Total     float64
+	}
+	var sums []inventorySum
+	s.db.Model(&model.Inventory{}).
+		Select("product_id, COALESCE(SUM(quantity), 0) AS total").
+		Where("shop_id = ? AND warehouse_id = ? AND product_id IN ? AND deleted_at IS NULL",
+			shopID, warehouseID, productIDs).
+		Group("product_id").Scan(&sums)
+
+	sumMap := make(map[uint64]float64, len(sums))
+	for _, s := range sums {
+		sumMap[s.ProductID] = s.Total
+	}
+
+	for _, item := range items {
+		have := sumMap[item.ProductID]
+		if have < item.Quantity {
+			var p model.Product
+			s.db.Where("id = ?", item.ProductID).First(&p)
+			name := p.Name
+			if name == "" {
+				name = fmt.Sprintf("商品ID %d", item.ProductID)
+			}
+			return fmt.Errorf("库存不足：%s 当前库存 %.0f，需要 %.0f", name, have, item.Quantity)
+		}
+	}
+	return nil
+}