WASHINGTON — Mayor Zohran Mamdani’s silver-spoon socialist adviser, Morris Katz, rushed over to beleaguered Graham Platner’s house Wednesday as they plotted how to remain a powerbroker in the Senate race — enraging Democrats. The gathering included Platner’s top campaign brass in addition to Katz, who is trying to hatch a plan for Platner to “remain a voice” in the Senate contest “no matter what” the accused rapist decides, a source familiar with deliberations told The Post.Platner has so far refused to step down as Maine’s Democratic Senate nominee after he was accused of rape by an ex-girlfriend — with the...