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/handler/admin.go

@@ -119,8 +119,14 @@ func (h *AdminHandler) ReconcileInventory(c *gin.Context) {
 
 	var invRows []invRow
 	var logRows []logRow
-	invQuery.Scan(&invRows)
-	logQuery.Scan(&logRows)
+	if err := invQuery.Scan(&invRows).Error; err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "inventory query failed: " + err.Error()})
+		return
+	}
+	if err := logQuery.Scan(&logRows).Error; err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "log query failed: " + err.Error()})
+		return
+	}
 
 	// 合并为 map 再求差
 	type key struct{ S, W, P uint64 }