security: fix GoogleLoginView audience check + replace Clerk with direct GIS flow

- verify_oauth2_token now passes GOOGLE_CLIENT_ID as third arg (audience check)
- fail-closed: returns 503 if GOOGLE_CLIENT_ID env var is not set
- add GOOGLE_CLIENT_ID = os.environ.get('GOOGLE_CLIENT_ID', '') to settings
- replace ClerkLoginViewTests with GoogleLoginViewTests (4 cases)
- update requirements-docker.txt

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

requirements-docker.txt

@@ -7,3 +7,4 @@ gunicorn==21.2.0
 django-extensions==3.2.3
 psycopg2-binary==2.9.9
 djangorestframework-simplejwt==5.3.1
+google-auth>=2.0.0