It turns out Mayor Zohran Mamdani’s East Harlem grocery store project is an even bigger boondoggle than we thought. A Post Business exclusive this week revealed that the La Marqueta site where Mamdani’s plans to spend $30 million was already approved for a $25 million upgrade nine years ago — meaning the taxpayers will burn $55 million on this white elephant. Plus operating costs that the mayor hasn’t even pretended to estimate yet. And in a neighborhood that already hosts ample food markets