Credentials and availability for the social sign-in providers.

A provider is live when its credentials are set and its <provider>_enabled setting has not been switched off. enabled/0 drives everything that varies by provider — the sign-in buttons, the /auth/:provider routes, and the GET /api/v1/auth/providers listing — so they can never disagree.

The id and secret are declared as a pair, so half-configuring one is a warning rather than a silent failure at the first login attempt.