After the ES256 upgrade, create_access_token requires non-empty
jwt_private_key / jwt_public_key. The auth_user, second_auth_user,
and admin_user fixtures in conftest.py call create_access_token
without keys being set, breaking all tests that exercise the auth
system. Add a function-scoped autouse fixture that provisions a
generated P-256 keypair for every test.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>