The Boston Archdiocese is pouring $850,000 into a last-minute effort to defeat a state ballot measure to legalize marijuana, calling increased drug use a threat to the Catholic Church’s health and social-service programs. The church’s contribution represents about a 50 percent increase over what the antimarijuana Campaign for a Safe and Healthy Massachusetts has collected so far. The total, however, is still less than half of what has been raised by the referendum’s proponents.