A wave of car bombs struck mainly in Shiite neighborhoods of Bagdad on Monday morning, killing at least 51 people and wounding dozens more, officials said, the latest in relentless violence roiling Iraq in recent months. The country’s interior ministry blamed al-Qaida-linked insurgents, saying they are exploiting the political infighting and security shortcomings to stage attacks. The deadliest of the day’s bombings was in the eastern Sadr City district, where a parked car bomb tore through a small vegetable market and its parking lot, killing seven people and wounding 16, a police officer said.