Add POST /auth/register: creates a non-superuser then auto-logs in,
returning the same TokenResponse as login. Gated by the new
allow_registration setting (env ALLOW_REGISTRATION, default true);
when disabled it raises PermissionDeniedError (403). Accounts remain
admin-only for superusers.
Tests cover create+login, duplicate (409), short password (422), and
the disabled (403) path.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Senko-san
olly