CI exposed that getFrontendBaseUrl() returns '' without FRONTEND_URL or
the general_site_url setting (local runs were masked by backend/.env):
the flow then sent a RELATIVE redirect_uri to the IdP, which surfaced
as an opaque IdP-side error. getRedirectUri now throws OIDC_BAD_CONFIG
with an actionable message (login route maps it to sso_error=config);
the settings GET degrades to an empty redirect_uri instead of 500ing.
The test pins FRONTEND_URL explicitly so it runs identically with and
without a local .env.
Paul Nothaft
ac1838fbd7