It’s a young composer’s opera about young love, performed by young singers. But for all its optimism, the production also represents a sad and unexpected ending. Mozart’s one-act ”Bastien und Bastienne” tells the story of a young shepherd and shepherdess whose romance gets a boost from the village magician. Opera Boston will present it twice at First Night on Saturday at Emmanuel Church, promising fun for all ages. But these will be the last performances for Opera Boston, whose board announced it was shutting down.