Katy Perry and Justin Trudeau just checked off another relationship milestone ... making their red-carpet debut as a couple at the Tribeca Festival in New York City. The singer and the former Canadian prime minister stepped out together Monday night for the premiere of her concert film, "Katy Perry: The Lifetimes Tour - Live From Paris," posing side by side for photographers in their first official red carpet appearance as a pair. The debut comes nearly a year after the couple first got