About nine hours after the final wave of the Boston Marathon began Monday, a group gathered alongside the course, now illuminated by streetlights, to celebrate the last person to finish among the roughly 29,000 competitors. The spectators shook cow bells and cheered as Janice “Rbey” Thomas sprinted past the finish line and a few feet beyond to ensure she had clocked all 26.2 miles. Then a race official placed a gold medal around her neck.