WASHINGTON — Braced for a fight, President Barack Obama on Wednesday unveiled the most sweeping proposals for curbing gun violence in two decades, pressing a reluctant Congress to pass universal background checks and bans on military-style assault weapons and high-capacity ammunition magazines like the ones used in the Newtown, Conn., school shooting. A month after that horrific massacre, Obama also used his presidential powers to enact 23 measures that don’t require the backing of lawmakers. The president’s executive actions include ordering federal agencies to make more data available for background checks, appointing a director of the Bureau of Alcohol, Tobacco, …