South Korea reported more than 800 new coronavirus cases on Saturday, the most of any country caught up in the epidemic, as China said it crossed a key threshold in its fight against the deadly disease. The increase in South Korea came mostly in Daegu, the southern city at the center of the country’s outbreak; in addition, four people have died. It took the total number of cases in the country to 3,150, according to Korea’s Centers for Disease Control and Prevention. …