test: add auth endpoint tests for single/multi mode
- 14 tests covering auth flow in both modes:
- TestSingleMode: health shows single, /me returns null user, clips accessible without auth
- TestMultiModeAuth: health shows multi, /me requires auth, clips require auth,
admin login, wrong password, nonexistent user, valid token /me,
clips with valid token, register with invite, invalid invite, duplicate username
- Tests require librosa (same as existing test_api.py) — pass in Docker CI
- Conftest fixture pattern matches existing app_client fixture
rin.kagamine
674abbbe8f