South Korea, which largely succeeded in quelling the spread of the coronavirus, is back on the defensive, with Seoul’s bars and clubs ordered closed, as the country reported its biggest one-day increase in new infections in a month. More than 50 cases have been linked to a 29-year-old man who, in a single night last weekend, visited five clubs and bars in a popular Seoul neighborhood, health officials said. He tested positive on Wednesday—the same day the South Korean government rolled out relaxed social-distancing…