Pin bcrypt to 4.0.1, add celery_app entry point

backend/app/workers/celery_app.py

@@ -0,0 +1,7 @@
+"""
+Celery entry point. Re-exports celery_app from tasks so the worker
+can be started with: celery -A app.workers.celery_app worker
+"""
+from app.workers.tasks import celery_app
+
+__all__ = ["celery_app"]