Add option for new questions to be unlisted by default in Admin

## Is your enhancement request related to a problem? Please describe

I want new questions to be initially unlisted by default. This differs from the review feature because questions in review cannot be answered. I want these unlisted questions to be answerable, specifically by administrators, allowing them to decide which questions should be made public.

## Describe the solution you'd like

Provide an option in the Admin settings to make new questions unlisted by default.

The issue names no files, tests, or entry points. Start by tracing the Admin settings flow and the question-creation visibility behavior; done means administrators can enable an option that leaves new questions unlisted but answerable, while allowing them to make selected questions public.

Written by the indexing model from the issue text.