feat(backends+domains): HTTP-Protokoll-Flags — force_http1 + disable_h3

Backend: force_http1 (bool, default false) — zwingt HAProxy auf der
Backend-Verbindung zu HTTP/1.1 statt h2,http/1.1 zu verhandeln.
Nötig für Legacy-Apps die kein h2 sprechen.

Domain: disable_h3 (bool, default false) — unterdrückt Alt-Svc-
Response-Header für diese Domain. Browser erhalten keinen h3/QUIC-
Hinweis und bleiben auf h2/http1.1.

Migration 0027, Model+Service+HAProxy-Template+UI+i18n.

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

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

@@ -20,13 +20,13 @@ interface Backend {
   id: number; name: string; scheme: string
   health_check_path?: string | null
   lb_algorithm: 'roundrobin' | 'leastconn' | 'source'
-  websocket: boolean; active: boolean
+  websocket: boolean; force_http1: boolean; active: boolean
 }
 interface BackendFormValues {
   name: string; scheme: 'http' | 'https'
   health_check_path?: string
   lb_algorithm: 'roundrobin' | 'leastconn' | 'source'
-  websocket: boolean; active: boolean
+  websocket: boolean; force_http1: boolean; active: boolean
   domain_ids?: number[]
 }
 interface BackendServer {
@@ -157,6 +157,7 @@ export default function BackendDetailPage() {
                 health_check_path: backend.health_check_path ?? undefined,
                 lb_algorithm: backend.lb_algorithm,
                 websocket: backend.websocket,
+                force_http1: backend.force_http1,
                 active: backend.active,
                 domain_ids: attached.map(d => d.id),
               }}
@@ -184,6 +185,10 @@ export default function BackendDetailPage() {
                 extra={t('backends.websocketHint')}>
                 <Switch />
               </Form.Item>
+              <Form.Item label={t('backends.forceHttp1')} name="force_http1" valuePropName="checked"
+                extra={t('backends.forceHttp1Hint')}>
+                <Switch />
+              </Form.Item>
               <Form.Item label={t('backends.active')} name="active" valuePropName="checked">
                 <Switch />
               </Form.Item>