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/src/routes/adminSettings.js

@@ -390,6 +390,10 @@ router.get('/sso', adminAuth, requirePermission('settings.view'), async (req, re
   try {
     const oidcService = require('../services/oidcService');
     const cfg = await oidcService.getOidcConfig();
+    // No public base URL configured → surface an empty redirect_uri rather
+    // than failing the whole settings read; the login route refuses to start
+    // the flow in that state anyway (OIDC_BAD_CONFIG).
+    const redirectUri = await oidcService.getRedirectUri().catch(() => '');
     res.json({
       oidc_enabled: cfg.enabled,
       oidc_issuer_url: cfg.issuerUrl || '',
@@ -399,7 +403,7 @@ router.get('/sso', adminAuth, requirePermission('settings.view'), async (req, re
       oidc_default_role: cfg.defaultRole,
       oidc_button_label: cfg.buttonLabel || '',
       oidc_scopes: cfg.scopes,
-      redirect_uri: await oidcService.getRedirectUri(),
+      redirect_uri: redirectUri,
     });
   } catch (error) {
     logger.error('Failed to read SSO settings', { error: error.message });

backend/src/services/oidcService.js

@@ -153,6 +153,14 @@ async function getClient(cfg) {
 async function getRedirectUri() {
   const { getFrontendBaseUrl } = require('../utils/frontendUrl');
   const base = (await getFrontendBaseUrl()).replace(/\/$/, '');
+  if (!base) {
+    // Without a public base URL the redirect_uri would be relative — the IdP
+    // would reject it with an opaque error on ITS side. Fail here with a
+    // clear config message instead.
+    const err = new Error('FRONTEND_URL (or the general_site_url setting) must be set for SSO');
+    err.code = 'OIDC_BAD_CONFIG';
+    throw err;
+  }
   return `${base}/api/auth/admin/sso/callback`;
 }