By Parisa Hafezi DUBAI (Reuters) -Protests in Iran entered a more violent phase on Sunday as students, who defied an ultimatum by the Revolutionary Guards and a warning from the president, were met with tear gas and gunfire from security forces, social media videos showed. The confrontations at dozens of universities prompted the threat of a tougher crackdown in a seventh week of demonstrations sparked by the death of 22-year-old Mahsa Amini after she was arrested by the morality police for attire deemed inappropriate. “Security is the red line of the Islamic Republic, and we will not allow th…