Multi-provider OIDC sign-in with Google and generic providers

You can sign in to Hive through Google or any standard OpenID Connect provider, and either can be configured together. The login screen shows one button per enabled provider, with Google using hardcoded discovery endpoints and generic OIDC discovered from HIVE_OIDC_ISSUER. Each provider supports an optional email-domain allowlist that blocks sign-in from accounts outside the allowed domains, and Google picks up the hosted-domain hint when a single domain is configured.