HAVANA — Cubans were glued to their televisions on Monday, many watching in a state of shock as President Raul Castro faced tough questions from American journalists who challenged him to defend Cuba’s record on human rights and political prisoners. In a country where publicly questioning the authority of Castro and his brother and predecessor Fidel is unthinkable for most, and where the docile state-run media almost always toe the party line, the live broadcast was must-see TV.