CIUDAD JUÁREZ, Mexico — Margelis Tinoco Lopez arrived at the border at 4 a.m. Monday for her 1 p.m. immigration appointment along with her husband and her 13-year-old son. Standing on the bridge in below-freezing weather, Lopez got an email from U.S. Customs and Border Protection that made her heart drop: “Existing appointments scheduled through the CBP One application are no longer valid.”