The families of recent Bridgeport murder victims are calling for unity on the eve of a major anti-violence summit. The meeting is scheduled to take place tomorrow at the Cathedral of the Holy Spirit, with more than 700 attendees expected. One attendee, Sandra Gallup, says her 17-year-old son was gunned down earlier this month. She says police were out in force only after his murder and that more needs to be done to save the city's youth. The summit will begin tomorrow at 10 a.m. and will run until 4 p.m. Several members of the state's congressional delegation have said they'll attend. Anti-violence forum held in Stamford