MIAMI — The Biden administration is expected to announce a partial staffing increase in the U.S. Embassy in Havana that will help address a yearslong visa backlog affecting thousands of Cuban American families, a source familiar with the plan said. Last month, Assistant Secretary of State Brian Nichols told lawmakers the plan involved sending temporary duty officers to Havana to help with immigration visa processing. The top U.S. diplomat in Havana, Timothy Zuñiga-Brown, is expected to make the formal announcement in a news conference Thursday in Havana. Still, the plan does not involve fully …