America’s largest owner of local sports channels is heading toward a complex $8.6 billion debt restructuring in bankruptcy court as it stakes its future on a new direct-to-consumer streaming service. After leveraging up to buy regional sports networks from Walt Disney Co. in 2019, Diamond Sports Group LLC is suffering from a decline in cable-TV subscribers, spurring negotiations with creditors and major sports leagues about its viability as a going concern. The outcome will have serious implications for the $55 billion world of sports-media rights: the company’s channels showcase Major League Baseball, National Basketball Association and National Hockey League games to fans from Detroit and Phoenix to San Diego.