For a century or more, the Mikhailovsky Theatre in St. Petersburg has been considered a second-tier institution, far below the level of its neighbor the Mariinsky or the Bolshoi in Moscow. Now oligarch Vladimir Kekhman, who made his fortune importing fruit, is pouring millions into the company, funding renovations and high-profile hires such as choreographer Nacho Duato and the Bolshoi Ballet’s star couple.