Detroit officials were cautiously declaring victory in their all-out campaign to dampen the annual Devil’s Night arson spree. Aided by sub-freezing temperatures, an army of 20,000 volunteers and city employees and a curfew for juveniles, fires during the pre-Halloween weekend dropped for the fourth year in a row, officials said. Although specific figures were not available, the number of fires Sunday--Devil’s Night--was running only slightly higher than a normal 24-hour period. Last year, 290 fires were reported in the three-day period ending Oct. 31. The city has about 210 fires in an average three-day period.