WASHINGTON — Florida business owner Robert Palmer cheered on the violence at the U.S. Capitol on Jan. 6 before he joined the fray. Screaming obscenities, he hurled a wooden plank and a fire extinguisher at police officers trying to ward off the mob.Nearly a year later, Palmer fought back tears when he faced the federal judge who sentenced him to more than five years in prison. He said he was “horrified, absolutely devastated” by what he had done.“I’m just so ashamed that I was a part of that,” Palmer told U.S. District …