Server-side authorization-code flow: /v1/auth/google redirects to the
consent screen with a CSRF state cookie; /v1/auth/google/callback
exchanges the code, validates the ID token (iss/aud/exp/email_verified),
and mints a 30-day session via upsertOAuthLogin. /v1/auth/providers lets
the login UI hide the button until GOOGLE_OAUTH_ID/SECRET are set.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
