fix(backend): 架构质量改进批次三 (#41-44)

- #41 CheckInventoryAvailability Scan 加错误检查，DB 故障不再误报库存不足
- #42 finance ListRecords 和 inventory Logs 补加 pageSize 上限（500）
- #43 ReconcileInventory 两条 Scan 加错误检查，DB 故障返回 500 而非误导性空结果
- #44 license Info 改用 CountDevices(licenseID) 统计当前 license 绑定数，修复跨 license 计数偏差

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

backend/internal/service/license.go

@@ -118,6 +118,13 @@ func (s *LicenseService) ShopInfo(shopID uint64) (*model.License, error) {
 	return &lic, nil
 }
 
+// CountDevices 返回指定 license 下已绑定设备数。
+func (s *LicenseService) CountDevices(licenseID uint64) (int64, error) {
+	var count int64
+	err := s.db.Model(&model.LicenseDevice{}).Where("license_id = ?", licenseID).Count(&count).Error
+	return count, err
+}
+
 // ListDevices 列出许可证下所有已绑定设备。
 func (s *LicenseService) ListDevices(shopID uint64) ([]model.LicenseDevice, error) {
 	var devs []model.LicenseDevice

backend/internal/service/stock.go

@@ -305,11 +305,13 @@ func (s *StockService) CheckInventoryAvailability(shopID, warehouseID uint64, it
 		Total     float64
 	}
 	var sums []inventorySum
-	s.db.Model(&model.Inventory{}).
+	if err := 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)
+		Group("product_id").Scan(&sums).Error; err != nil {
+		return err
+	}
 
 	sumMap := make(map[uint64]float64, len(sums))
 	for _, s := range sums {