In the blink of an eye, it was over. Justin Timberlake may want to take notice. Cpl. Kelsey De Santis, the Marine who famously invited the singer and actor to attend the Marine Corps birthday ball with her, competed Saturday night in Operation Octagon XVI, a mixed martial arts event in Sterling, Va. To say it was a one-sided fight would be an understatement. De Santis, fighting in the 145-pound featherweight title fight, pummeled her opponent, Stacy Sneeringer. With a flurry of energy, De Santis took Sneeringer to the mat early, and finished her off 2:51 into the fight with…