A great city is under siege, and as the bullets fly, the defenders of Leningrad use loudspeakers to blast Shostakovich’s Seventh Symphony, defiant and colossal, at the Nazi forces. Everything about the Leningrad symphony is incredible, and it’s never more astonishing than when you hear it performed live. As Vasily Petrenko begins his last season in Liverpool as Chief Conductor, he salutes his home city