Add support for running behind a reverse proxy

I deployed this behind a load balancer, configured with TLS, etc, etc. An issue I found when logging in was the app specifies the wrong scheme in the `redirect_uri=http` when calling the IdPs `/authorize` endpoint.

One solution would be to modify the `url_for`s in `src/auth_playground/oauth.py` to set `_scheme="https"`. A potential other alternative would be to use the [ProxyFix](https://werkzeug.palletsprojects.com/en/stable/middleware/proxy_fix/#werkzeug.middleware.proxy_fix.ProxyFix) middleware.

Start in src/auth_playground/oauth.py and trace the url_for calls that build the IdP /authorize redirect_uri. Compare setting _scheme="https" with adding Werkzeug ProxyFix, using the linked ProxyFix documentation as guidance. Done means the redirect_uri uses https when the app runs behind a TLS-terminating load balancer.

Written by the indexing model from the issue text.