Single configurable OIDC provider with Google preset

Hive now supports a single OIDC provider per instance, configurable via HIVE_OIDC_PROVIDER=google or HIVE_OIDC_PROVIDER=generic. Choosing Google fills in Google’s hardcoded endpoints and applies the hd= domain hint when a single allowed domain is set, while generic OIDC lets operators supply their own authorize, token, and userinfo URLs. The shared HIVE_OIDC_CLIENT_ID, HIVE_OIDC_CLIENT_SECRET, and HIVE_OIDC_ALLOWED_DOMAINS variables simplify setup and keep the Helm chart provider-agnostic. The callback path remains /auth/<provider>/callback.