Remove FastAPI auth-lib SQLAlchemy implementation in favour of psycopg

[spwoodcock]

• We have both a sqlalchemy and psycopg implementation for fastapi, one labelled _psycopg.
• We don't need both, as sqlalchemy will be using psycopg anyway (we don't support mysql and other drivers), so it makes more sense to keep the less abstracted psycopg implementation only (in line with the new LiteStar impl too).

[spwoodcock]

Doing this would also remove this warning I get from pytest in projects containing this package:

opt/python/lib/python3.11/site-packages/hotosm_auth_fastapi/db_models.py:34
  /opt/python/lib/python3.11/site-packages/hotosm_auth_fastapi/db_models.py:34: MovedIn20Warning: The ``declarative_base()`` function is now available as sqlalchemy.orm.declarative_base(). (deprecated since: 2.0) (Background on SQLAlchemy 2.0 at: https://sqlalche.me/e/b8d9)
    Base = declarative_base()