fix(backend): 补全多处 Scan/Count 错误未检查，私钥缺失改为 Fatal

- stock.go: ApproveStockIn 入库前库存 Scan 加错误检查
- license.go: Activate 阶段 Count 设备数加错误检查；createTrialLicense
  私钥未配置由静默跳过改为 log.Fatalf，防止生产环境新注册门店无 license
- inventory.go: List 接口两处 Raw Scan 加错误检查并返回 500
- finance.go: ArBalance 汇总 Raw Scan 加错误检查并返回 500

backend/internal/service/stock.go

@@ -52,10 +52,12 @@ func (s *StockService) ApproveStockIn(shopID, orderID, reviewerID uint64) error
 
 			// 计算入库前库存总量（用于流水记录）
 			var qtyBefore float64
-			tx.Model(&model.Inventory{}).
+			if err := tx.Model(&model.Inventory{}).
 				Where("shop_id = ? AND warehouse_id = ? AND product_id = ? AND deleted_at IS NULL",
 					shopID, warehouseID, productID).
-				Select("COALESCE(SUM(quantity), 0)").Scan(&qtyBefore)
+				Select("COALESCE(SUM(quantity), 0)").Scan(&qtyBefore).Error; err != nil {
+				return err
+			}
 
 			var unitPricePtr *float64
 			if itemCopy.UnitPrice != 0 {
@@ -322,7 +324,7 @@ func (s *StockService) CheckInventoryAvailability(shopID, warehouseID uint64, it
 		have := sumMap[item.ProductID]
 		if have < item.Quantity {
 			var p model.Product
-			s.db.Where("id = ?", item.ProductID).First(&p)
+			s.db.Where("id = ?", item.ProductID).First(&p) //nolint:errcheck — best-effort name lookup for error message
 			name := p.Name
 			if name == "" {
 				name = fmt.Sprintf("商品ID %d", item.ProductID)