First, they came for the mailboxes.The once-fantastical notion of a political or military coup in the United States has long lingered in the American imagination. Older boomers might remember, for example, the book and movie “Seven Days in May.” It’s a riveting plotline because, in a nation with a 231-year tradition of peacefully transferring power, a coup was always something that Can’t Happen Here. Americans’ knowledge of how coups even work comes mostly from stories on NPR from faraway lands where soldiers seize a nation’s key chokepoints, as tanks roll onto the tarmac of the international …