Configurable OIDC login with Google or a generic provider

You can configure Hive to sign users in through a single OIDC provider. Use HIVE_OIDC_PROVIDER=google for a Google preset that only needs client credentials, or use HIVE_OIDC_PROVIDER=generic with your own issuer, authorize, token, and userinfo URLs. Provider-specific buttons appear on the login page and domain allowlists are enforced during the callback.