STOCKHOLM — Paris, Copenhagen, Brussels. In just over a year three European capitals have been ravaged by bombs and gunfire. After each attack life slowly returns to normal. But it’s a new normal for Europe, where terror alerts are always on high and where people in cities so far spared major violence assume it’s a matter of when, not if. “With each atrocity that occurs we change,” said Ian Duncan, a Scottish member of the European Parliament in Brussels.