Her voice was ineffably moving, her appearances were the stuff of legend. She gave nearly 600 performances over an 18-year span. In one appearance, she received 27 curtain calls in an outpouring of adoration lasting more than 40 minutes. For many opera lovers, there is and always will be only one true diva: Maria Callas, known to her legions of fans simply as “La Divina.” A special exhibition devoted to Callas arrives in S.F. this month.