Calvin Lee had his pick of jobs. A former silver medalist in the International Olympiad in Informatics, Lee had racked up internships at prominent institutions from Jane Street to Google during his three years at MIT. By the spring of 2019, he was juggling offers from Citadel and other hedge funds that would have made him a multimillionaire before he was old enough to rent a car. But then Lee got a call from Karim Atiyeh, his former boss at a tiny fintech startup called Paribus, where he had int...