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/inventory.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/wangjia/jiu/backend/internal/middleware"
 	"github.com/wangjia/jiu/backend/internal/model"
+	"github.com/wangjia/jiu/backend/internal/util"
 )
 
 type InventoryHandler struct {
@@ -128,6 +129,7 @@ func (h *InventoryHandler) Logs(c *gin.Context) {
 	shopID := middleware.GetShopID(c)
 	page, _ := strconv.Atoi(c.DefaultQuery("page", "1"))
 	pageSize, _ := strconv.Atoi(c.DefaultQuery("page_size", "20"))
+	pageSize = util.ValidatePageSize(pageSize, 20, 500)
 
 	query := h.db.Model(&model.InventoryLog{}).Where("shop_id = ?", shopID)