feat(backend): 库存列表列头排序——库存/成本价/总价/生产日期/入库时间全局排序

sort_by 白名单列映射 SQL 表达式（cost/price/prod_date 与 SELECT 同 COALESCE
口径，排序与显示一致）+ sort_dir，非法值忽略回默认；id 兜底保证分页稳定。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JJ1g8XV1YhhmHRzhwWEW7o

backend/internal/handler/inventory.go

@@ -158,6 +158,23 @@ func (h *InventoryHandler) List(c *gin.Context) {
 		return
 	}
 
+	// 排序（2026-07-07 列头排序）：白名单列 → SQL 表达式，防注入；带 id 兜底保证分页稳定。
+	// cost/price/prod_date 用与 SELECT 相同的 COALESCE 口径，排序结果与显示一致。
+	orderExpr := "inv.id DESC"
+	if sortCol, ok := map[string]string{
+		"qty":       "inv.quantity",
+		"cost":      "COALESCE(sii.cost_price, inv.unit_price, p.purchase_price)",
+		"price":     "inv.quantity * COALESCE(sii.cost_price, inv.unit_price, p.purchase_price)",
+		"prod_date": "COALESCE(DATE(sii.production_date), DATE(inv.production_date))",
+		"in_time":   "inv.created_at",
+	}[c.Query("sort_by")]; ok {
+		dir := "ASC"
+		if c.Query("sort_dir") == "desc" {
+			dir = "DESC"
+		}
+		orderExpr = sortCol + " " + dir + ", inv.id DESC"
+	}
+
 	// Data query
 	dataSQL := `
 		SELECT
@@ -183,7 +200,7 @@ func (h *InventoryHandler) List(c *gin.Context) {
 		LEFT JOIN products p ON p.id = inv.product_id AND p.deleted_at IS NULL
 		LEFT JOIN warehouses w ON w.id = inv.warehouse_id
 		WHERE ` + baseWhere + `
-		ORDER BY inv.id DESC
+		ORDER BY ` + orderExpr + `
 		LIMIT ? OFFSET ?`
 
 	dataArgs := append(args, pageSize, (page-1)*pageSize)