Investigate user registration setup

This card is a preface to all user sign-in work and is intended as an investigative card so we're informed enough to discuss potential user registration options in an upcoming client meeting. Right now, we're assuming that we will refactor the email trigger on sign-in so that users only receive an email with a valid session link if (1) their email is in our User table and (2) that user has the `has_voucher` value set to `True`.

We need to determine:
- How the current login/logout authentication works, and if we can continue with the existing endpoints
- If we can easily refactor authentication logic to check for `has_voucher` as a means to validate ECHO program users
- If there's a simple means for counselors to register new ECHO program participants. One example may be to bulk upload new user emails via the django admin (the `has_voucher` field would need to default for `True` for these new users).

Artifacts from this issue should be: 1) ADR on current authentication system 2) Documented options to present to BHA EHCO on what user registration workflow makes sense with their program

We should also clarify in client meeting if we should keep non-voucher holder users in the database moving forward (or maybe move them to archived users)

Start by tracing the current login/logout authentication endpoints and email trigger, then inspect how the User table and Django admin support registration. Produce an ADR describing the current authentication system and document registration options, including bulk email upload and the treatment of non-voucher users, for the client meeting.

Written by the indexing model from the issue text.