Nidia Montenegro fled violence and poverty at home in Venezuela, survived a kidnapping as she traveled north into Mexico, and made it to the border city of Tijuana on Sunday for a U.S. asylum appointment that would finally reunite her with her son living in New York. That appointment is now canceled. As President Donald Trump declared a national emergency at the southern border, migrants waiting in Mexico nervously checked the U.S. government app known as CBP One, by which many have been able to schedule appointments to claim asylum. As they refreshed the app, an alert came through: "Existing...