Eurovision ‘s theme, “United by Music ,” has rarely sounded more ironic as Vienna gears up to host the kitschy song contest. On a boiling day a week ago, the city’s main stadium appeared more like a small military installation: new security cameras peered into the neighborhood — Vienna’s poorest — as staffers in lanyards sat in fold-out chairs, guarding every conceivable entrance. Fencing featuring the titles of past Eurovision hits — Rise Like a Phoenix, Wasted Love — rose from concrete traffic barriers: half-advertisement, half-warning.   The concert hall, called Stadthalle, occupies a city block and is surrounded by residential buildings. It’s in a neighborhood with paltry access to parks, and now the green space around the Stadthalle is being sequestered as well, depriving residents of 30,000 square meters of parkland in the name of European unity.…