feat(backend): 授权订单列表接口——分页/筛选/汇总，支撑订单管理 tab

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/router/router.go

@@ -124,6 +124,7 @@ func Setup(r *gin.Engine, db *gorm.DB) {
 		license.GET("/purchase/:out_trade_no", payH.PurchaseStatus)
 		license.POST("/purchase/:out_trade_no/cancel", payH.Cancel)
 		license.GET("/promo-status", payH.PromoStatus)
+		license.GET("/purchases", payH.Purchases)
 	}
 
 	// 业务路由：ReadOnly + LicenseGuard（过期只读/锁定拦截写操作）