Andrea Kalin’s short documentary “Public Defender” follows the work of Heather Shaner, a criminal-defense lawyer who has represented 42 January 6th defendants so far. The Department of Justice has brought close to 1,500 such cases. More than 300 are still pending—including, of course, the case against Donald Trump. The legal response to the insurrection at the U.S. Capitol was, in some ways, an exercise in improvisation in the aftermath of an unprecedented event—what amounted to an attempted cou