Make Redis an optional dependency

At the moment Redis is a shared database, used so that the Admin app can configure the Protected app.

We don't have to use Redis though, we could use any SQL database table.

Let's make Redis an optional dependency.

Start by tracing how Redis is shared between the Admin app and the Protected app, then identify the configuration and persistence boundaries involved. Determine how an SQL database table could replace Redis and how the dependency remains optional; done means the apps can coordinate without requiring Redis.

Written by the indexing model from the issue text.