Discovery’s cable deal rests on some rosy assumptions. The producer of “Shark Week” is buying Food Network and HGTV owner Scripps Networks Interactive for $14.6 billion. Cost savings won’t cover the premium and getting distributors to pony up for more content will be a hard sell in a churning media market. Like all deals that involve media mogul John Malone, who holds a voting stake in Discovery, this one is thorny. For starters, Discovery and Scripps have been mulling a merger for years. Any tie-up requires wooing the Scripps family, which holds more than 90 percent of the vote. Discovery