Multi-provider OIDC authentication with Google built in

Hive now supports multiple OIDC providers out of the box, including Google and any generic provider. The login page renders a button for each configured provider, and the callback flow runs at /auth/:provider. For Google, allowed domains can be restricted with HIVE_GOOGLE_ALLOWED_DOMAINS or HIVE_OIDC_ALLOWED_DOMAINS, and a single configured domain is passed as Google’s hosted-domain hint to pre-filter accounts. Operator-facing env vars were consolidated to a single HIVE_OIDC_PROVIDER=google|generic discriminator, while still supporting Google as a first-class option.