fix(config): handle comma-separated FRONTEND_URL for domains and CORS

FRONTEND_URL may hold a list of origins (e.g. CORS needs both
panel.abrban.com and abrban.com). The domain resolvers ran new URL()
on the whole string, so new URL('https://a,https://b').hostname became
"a,https" and leaked into ingress hosts, which k8s then rejected with
the generic "HTTP request failed" surfaced in the UI. CORS likewise
never split the list, so the second origin never matched.

Parse only the first URL for domain/preview-root resolution, and split
the list into an array for enableCors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/config/configuration.ts

@@ -1,5 +1,14 @@
+// FRONTEND_URL may hold a comma-separated list of origins (used for CORS).
+// Domain/host resolution must only ever look at a single URL, otherwise
+// `new URL('https://a.com,https://b.com')` yields the garbage hostname
+// "a.com,https" which then leaks into ingress hosts and gets rejected by k8s.
+function firstFrontendUrl(): string | undefined {
+  const first = (process.env.FRONTEND_URL || '').split(',')[0]?.trim();
+  return first || undefined;
+}
+
 function resolvePlatformDomainFromEnv(): string {
-  const frontendUrl = process.env.FRONTEND_URL;
+  const frontendUrl = firstFrontendUrl();
   if (frontendUrl) {
     try {
       const url = new URL(frontendUrl);
@@ -35,7 +44,7 @@ function resolvePreviewRootDomainFromEnv(): string {
     return explicit.trim().toLowerCase();
   }
 
-  const frontendUrl = process.env.FRONTEND_URL;
+  const frontendUrl = firstFrontendUrl();
   if (frontendUrl) {
     try {
       const url = new URL(frontendUrl);