TIJUANA, Mexico (AP) — A mobile app for migrants to seek asylum in the United States has been overwhelmed since it was introduced this month in one of several major changes to the government’s response to unprecedented migration flows. New appointments are made available daily. But migrants are increasingly frustrated by a variety of error messages. Many can’t log in. Others are hopeful when they get a date, only to be deflated when the screen freezes at final confirmation. The daily ritual resembles a race for concert tickets when online sales begin for a major act.