feat: per-Backend timeout server + Alerts-Deeplink + Retention + Cert-Prune — v1.3.4

- feat(backends): per-Backend `server_timeout_seconds` (nullable, Default 60s).
  Rendert `timeout server <N>s` im HAProxy-Backend-Block — für langsam
  antwortende Upstreams (KI-/Inferenz-Server mit gepufferter Antwort).
  Migration 0044 (+CHECK 1..86400), Model/Repo/Template/UI + Render-Test.
- fix(ui): Dashboard-Alert-Karte verlinkt auf /alerts?tab=events; Alerts-Seite
  respektiert ?tab= (Deeplink landete bisher auf leerem Channels-Tab).
- feat(scheduler): alert_events-Retention (90d) im täglichen Cleanup-Tick —
  Schutz vor unbounded growth der node-lokalen Health-Event-History.
- fix(cluster): Cert-Sync prunt jetzt lokale .pem die der Primary nicht mehr
  hat (Waisen gelöschter Domains); schützt _default.pem + eigenen Node-Cert.
- fix(security): x/text v0.37→v0.39 (GO-2026-5970, Infinite-Loop; via ACME+goose
  aktiv aufgerufen — govulncheck-Release-Gate).

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

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

@@ -20,13 +20,17 @@ interface Backend {
   id: number; name: string; scheme: string
   health_check_path?: string | null
   lb_algorithm: 'roundrobin' | 'leastconn' | 'source'
-  websocket: boolean; force_http1: boolean; active: boolean
+  websocket: boolean; force_http1: boolean
+  server_timeout_seconds?: number | null
+  active: boolean
 }
 interface BackendFormValues {
   name: string; scheme: 'http' | 'https'
   health_check_path?: string
   lb_algorithm: 'roundrobin' | 'leastconn' | 'source'
-  websocket: boolean; force_http1: boolean; active: boolean
+  websocket: boolean; force_http1: boolean
+  server_timeout_seconds?: number | null
+  active: boolean
   domain_ids?: number[]
 }
 interface BackendServer {
@@ -174,6 +178,7 @@ export default function BackendDetailPage() {
                 lb_algorithm: backend.lb_algorithm,
                 websocket: backend.websocket,
                 force_http1: backend.force_http1,
+                server_timeout_seconds: backend.server_timeout_seconds ?? undefined,
                 active: backend.active,
                 domain_ids: attached.map(d => d.id),
               }}
@@ -205,6 +210,11 @@ export default function BackendDetailPage() {
                 extra={t('backends.forceHttp1Hint')}>
                 <Switch />
               </Form.Item>
+              <Form.Item label={t('backends.serverTimeout')} name="server_timeout_seconds"
+                extra={t('backends.serverTimeoutHint')}>
+                <InputNumber min={1} max={86400} step={30}
+                  style={{ width: '100%' }} addonAfter="s" placeholder="60 (default)" />
+              </Form.Item>
               <Form.Item label={t('backends.active')} name="active" valuePropName="checked">
                 <Switch />
               </Form.Item>