The parent company of The San Diego Union-Tribune announced yesterday that it plans to sell the paper to a Beverly Hills private equity firm that specializes in turning around troubled ventures. The buyer, Platinum Equity, is a 14-year-old firm headed by billionaire investor Tom Gores. It has purchased companies in industries as diverse as information technology and metal processing. This is its first newspaper deal. A principal in the firm said newspapers were a logical target, given the steep challenges the industry faces, such as circulation declines and growing online competition. “There's no question that the newspaper industry is undergoing...