When the K-pop group BTS staged a show in South Korea that drew a crowd of 55,000, the police were ready, assigning 1,300 officers to keep people safe. And when political protests are held, however modest in size, the country’s police are famous for laying careful plans to make sure crowds do not get out of control. But that did not happen Saturday night, when tens of thousands of boisterous young Koreans, freed at last of pandemic restrictions, surged into a Seoul nightlife neighborhood to celebrate Halloween.