Three years ago, Harvard Street Neighborhood Health Center was bleeding money. Programs were cut, staff was barely paid, care deteriorated. Some of the city's poorest patients were forced to look elsewhere for medical help or do without. As the Dorchester facility struggled to keep its doors open, prosecutors say, a payroll consultant was living a life of luxury, with a penthouse in Cambridge and several sports cars, including a Lamborghini. Yesterday a Suffolk County grand jury indicted Nzeribe McKenzie, 33, of Cambridge, alleging that he manipulated the books to conceal thefts of some $750,000 from the health center from 2003 to 2008 and that he spent the money on himself, according to the Suffolk district attorney.