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/RoutingRules/index.tsx

@@ -270,6 +270,7 @@ export default function RoutingRulesPage() {
         onCancel={() => { setEditing(null); setCreating(false) }}
         onOk={() => { void form.submit() }}
         confirmLoading={create.isPending || update.isPending}
+        destroyOnHidden
       >
         <Form
           form={form}
@@ -285,7 +286,15 @@ export default function RoutingRulesPage() {
               placeholder={t('routing.selectDomain')}
             />
           </Form.Item>
-          <Form.Item label={t('routing.pathPrefix')} name="path_prefix" rules={[{ required: true }]}>
+          <Form.Item
+            label={t('routing.pathPrefix')}
+            name="path_prefix"
+            extra={t('routing.pathPrefixHint')}
+            rules={[
+              { required: true },
+              { pattern: /^\/[a-zA-Z0-9\-._~!$&'()*+,;=:@/%]*$/, message: t('routing.pathPrefixInvalid') },
+            ]}
+          >
             <Input placeholder="/" />
           </Form.Item>
           <Form.Item label={t('routing.backend')} name="backend_id" rules={[{ required: true }]}>