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 }

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
 	}

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)
 

backend/internal/handler/license.go

@@ -64,10 +64,11 @@ func (h *LicenseHandler) Info(c *gin.Context) {
 		return
 	}
 
-	var deviceCount int64
-	// count active devices for this shop
-	devs, _ := h.svc.ListDevices(shopID)
-	deviceCount = int64(len(devs))
+	deviceCount, err := h.svc.CountDevices(lic.ID)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		return
+	}
 
 	phase := middleware.CalcLicensePhase(lic.ExpiresAt)
 	c.JSON(http.StatusOK, gin.H{"data": gin.H{