More than 30% of Volkswagen AG workers at the German automaker’s only U.S. plant in Chattanooga, Tenn., have signed union authorization cards in less than a week, the United Auto Workers said in a news release Thursday. That includes more than 1,000 workers, according to the union, which last week formally launched a campaign to organize workers at 13 non-union automakers across the country. Hitting the milestone means the organizing drive is going public there. If 50% of workers sign authorization cards at the plant, the UAW says it will hold a rally there. If it receives 70% support, that’s …