BUDAPEST, Hungary — In Hungarian, there is an expression that anti-Semitism is like a hidden spring, often imperceptible yet omnipresent, a source that can be tapped by demagogues, polemicists, and politicians. It is a dangerous source, Adam Schönberger explains, for when it is tapped, it can cause a flood. Schönberger is a veteran Hungarian activist and co-founder of Aurora, a café and community center operated by Marom, a Jewish youth-group that he also directs. When we meet there on a Monday evening in late August, Aurora, located in Budapest’s Eighth District, is buzzing. Opposition activists type away on laptops in…..