LVIV, Ukraine — The Ukrainian city of Lviv, once a major center of Jewish life in Eastern Europe, commemorated on Sunday the 75th anniversary of the annihilation of the city’s Jewish population by Nazi Germany and honored those working today to preserve what they can of that vanished world. City authorities presented the honored recipients with 75 glass keys — replicas of a metal key that once belonged to a Jewish synagogue and which an American artist found at a street market in Lviv.