South Korea shut down the heart of Seoul on Saturday for a comeback concert by K-pop supergroup BTS, as authorities prepare for an estimated 260,000 fans to flood the city’s streets and millions more to watch the show live on Netflix. The one-hour-long concert, set to be held in the capital’s historic Gwanghwamun Square, marks the release of the seven-member group’s first new album in more than three years and the start of a global tour in April. BTS made their debut in 2013 and ‌have since gone...