OIDC login with Google and generic providers

Hive now supports OIDC-based login for both Google and any generic OpenID Connect provider. Configure HIVE_GOOGLE_CLIENT_ID, HIVE_GOOGLE_CLIENT_SECRET, and optional allowed domains, or point a generic provider at its issuer via HIVE_OIDC_ISSUER, HIVE_OIDC_CLIENT_ID, and HIVE_OIDC_CLIENT_SECRET. Each configured provider appears as its own button on the login screen, with the callback handled at /auth/:provider/callback.