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/finance.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/wangjia/jiu/backend/internal/middleware"
 	"github.com/wangjia/jiu/backend/internal/model"
+	"github.com/wangjia/jiu/backend/internal/util"
 )
 
 type FinanceHandler struct {
@@ -33,9 +34,7 @@ func (h *FinanceHandler) ListRecords(c *gin.Context) {
 		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
 		return
 	}
-	if q.PageSize <= 0 {
-		q.PageSize = 50
-	}
+	q.PageSize = util.ValidatePageSize(q.PageSize, 50, 500)
 	if q.Page <= 0 {
 		q.Page = 1
 	}