NEW YORK — Americans paused Thursday to celebrate their blessings despite terrorism fears and racial tensions over fatal police shootings across the country. Spectators at the annual Macy’s Thanksgiving Day Parade in New York noted a stepped-up police presence, with officers perched on buildings like Radio City Music Hall and watching from helicopters hovering overhead. “It’s a little scary, but at least it’s keeping us safe,” Kim Miller, of Boston, said of the heavy security.