Faruk Fatih Özer stood in front of a passport control officer at Istanbul Airport, a line of impatient travelers queuing behind him. He pulled his face mask below his chin for the security camera. Surely he was nervous. The 27-year-old had unruly black hair, a boy-band face, and a patchy beard. Normally he overcompensated for his callow features by dressing in a pressed three-piece suit. But this spring day he wore black trainers and a navy-blue sweater hastily pulled over a white polo shirt, as...