The first waves of arrests in the deadly siege at the U.S. Capitol focused on the easy targets. Dozens in the pro-Trump mob openly bragged about their actions on Jan. 6 on social media and were captured in shocking footage broadcast live by national news outlets.But six months after the insurrection, the Justice Department is still hunting for scores of rioters, even as the first of more than 500 people already arrested have pleaded guilty. The struggle reflects the massive scale of the investigation and the grueling work …