fix(oidc): fail clearly when no public base URL is configured

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.

backend/__tests__/integration/oidcSso.test.js

@@ -38,6 +38,10 @@ describe('OIDC SSO (#798)', () => {
 
   beforeAll(async () => {
     process.env.JWT_SECRET = process.env.JWT_SECRET || 'oidc-test-secret';
+    // The redirect_uri derives from the public base URL — pin it explicitly:
+    // CI has no backend/.env, and getFrontendBaseUrl() returning '' makes
+    // buildAuthorizationRequest fail (by design) with OIDC_BAD_CONFIG.
+    process.env.FRONTEND_URL = 'http://localhost:5199';
     ({ db, cleanup } = await bootCrmDb());
 
     idp = new MockOidcProvider();