Nearly a million people signed up for health insurance under President Barack Obama’s law even after the official enrollment season ended, helping push the share of uninsured Americans below 10 percent and underscoring how hard it could be for Republicans to dismantle the program. The Health and Human Services Department said Thursday that 943,934 new customers have signed up since open enrollment ended on Feb. 22, benefiting from “special enrollment periods” keyed to life changes and other circumstances.