Feature Request: OIDC oauth2 sign in / registration

## What is the problem that your feature request solves
SSO/ability to sign in and up with a OIDC identity provider.

## Describe the ideal specific solution you'd want, and whether it fits into any broader scope of changes
OIDC + scope mapping, where you can grant a user admin privileges if they have a matching key-value pair in the token

## What hacks or alternative solutions have you tried to solve the problem?
None yet, but I am looking at putting it behind oauth2-proxy

## How badly do you want this new feature?
It would be nice to have eventually

I like ArchiveBox and would be willing to contribute development time for this feature.

The issue names no files, tests, or entry points. First establish the project's current authentication and registration path; completion would require agreed OIDC sign-in and registration plus token key-value scope mapping for admin privileges, with tests.

Written by the indexing model from the issue text.