A bombshell memo made public Thursday proves the city knew about the potential risks of Sept. 11, 2001 toxins weeks after the terror attacks — as officials told New Yorkers it was safe to return to Lower Manhattan, local pols said. City Council Speaker Julie Menin and Councilwoman Gale Brewer (D-Manhattan) unveiled the October 2001 memo, in which Big Apple lawyers admitted the city could face tens of thousands of lawsuits, including from people exposed to toxins after being advised they could