Ability to create private posts/events

There are posts/events that the general public doesn't have to see

A `SchOnly` flag might suffice for the first iteration, but something more comprehensive will be needed later

Start by mapping where posts and events are represented and where their public visibility is decided; the issue names no files, tests, or entry points. Clarify the first-iteration meaning of the SchOnly flag and define done as restricting marked posts/events from the general public while preserving the existing public behavior.

Written by the indexing model from the issue text.