Banking woes followed a long era of easy money. : Each of the last four U.S. presidents has confronted an economic crisis serious enough to warrant extraordinary government intervention in the workings of the free market. Once rare, such dramatic rescues have become the norm. The authorities’ swift response this month to the collapse of Silicon Valley Bank, which until the day it failed had been regarded as of little importance beyond the technology sector, revealed a brittle system addicted to infusions of official support, according to some economists.