Env: APP_URL, and per provider OAUTH_GOOGLE_CLIENT_ID/_SECRET,
OAUTH_GITHUB_CLIENT_ID/_SECRET, OAUTH_FACEBOOK_CLIENT_ID/_SECRET.
Flow: GET /api/auth/oauth/{provider} -> callback -> session.
Package oauth adds OAuth2 social login (Google, GitHub, Facebook) to togo auth.
It registers /api/auth/oauth/{provider} + callback routes; on success it
find-or-creates the user via auth and issues a togo session. Depends on the
auth plugin. Install: `togo install togo-framework/auth-oauth`.