D.C. housing officials have routinely subsidized home purchases that low-income buyers could not afford, paving the way for foreclosures, liens and financial hardships. Nearly one in five buyers participating in the city’s 35-year-old loan program for first-time homeowners is behind on mortgage payments, city officials said — a default rate that’s at least three times higher than the overall rate in the region. Nearly 50 buyers have received notices of foreclosure in recent years, while more than 50 others have struggled with homeowner association or utility liens, The Washington Post has found.