fix(routing+backends): Pfad-Präfix-Validierung + Warnung bei leerem Backend-Pool (v1.1.141)

- RoutingRules: path_prefix Pattern-Validator (muss mit / beginnen) + destroyOnHidden
- Backends: Alert-Banner wenn aktives Backend 0 Server hat (→ HAProxy 503)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

management-ui/src/pages/Backends/index.tsx

@@ -329,6 +329,15 @@ export default function BackendsPage() {
           </Tag>
         )}
       />
+      {serverCounts && (data ?? []).some(b => b.active && (serverCounts[b.id] ?? 0) === 0) && (
+        <Alert
+          type="warning"
+          showIcon
+          className="mb-12"
+          message={t('backends.noServersWarning')}
+          description={t('backends.noServersWarningDesc')}
+        />
+      )}
       <DataTable
         rowKey="id"
         loading={isLoading}