Rain or shine, Isaura Mendes will visit the corner of Dudley and Wendover streets in Dorchester on Sunday evening to light a candle. She knows that spot all too well: It’s near where her son Alexander “Matthew’’ Mendes was felled by a bullet six years ago. The annual Community Peace Candlelight Vigil is part of Mendes’s mission to share a message of peace and forgiveness and to remember all victims of homicide in Boston.