The first rule of the pandemic: avoid a crowded house full of strangers. But for live theatres and music venues, a full house is the sign of an entertaining — and profitable — evening. While restaurants, retail shops and gyms have slowly reopened, indoor performance venues have kept their doors closed. As California nears June 15 — the finish-line for the state’s Blueprint for a Safer Economy — raising the curtains at these venues still has its challenges.