Alvin Kass, a revered rabbi who was the youngest chaplain in New York City Police Department history and who went on to become its longest-serving, died Wednesday in Manhattan. He was 89. His daughter, Sarah Kass, said the cause of his death, in a hospital, was complications of sepsis. Kass was the department’s chief chaplain and, after nearly 60 years, not just its longest-serving chaplain but also its longest-serving employee of any kind, Commissioner Jessica Tisch said.