feat: umfangreiches UI+API-Polish (v1.1.36–1.1.42)

Backend:
- Audit-Log: Search-Endpoint mit ILIKE-Filter (actor/action/subject/date)
- NTP: /ntp/status via chronyc tracking (Stratum, Offset, Quelle)
- System: /service-restart mit Allowlist (haproxy/squid/unbound/chrony/scheduler)
- Domain-Response-Headers + Rate-Limit (Migration 0024)
- Join-Tokens (Migration 0025), Cluster-mTLS, Aggregator-Fan-Out
- apt-Service für Update-Banner (apt-get update + Versionsprüfung)
- Backup-Retry mit exponential backoff (retry_apt 3×)
- publish.sh fail-fast + cleanup-old.sh (max 10 Versionen)

Frontend:
- Audit-Log-Page (/audit) mit Filter + Pagination
- ErrorBoundary an React-Root + Vite build-target festgenagelt (iOS 15+)
- Storage-Schema-Stamp: auto-wipe bei Versions-Mismatch (blank-page-Fix)
- EmptyState-Komponente überall ausgerollt
- SSL: Aggregate-Karte (total/expiring/expired/errors)
- Backups: Aggregate-Karte (letzter Backup/Größe/Fehlschläge 24h) + Backup-Now
- NTP: Sync-Status-Karte (chronyc tracking live)
- Domains: Backend-UP/DOWN-Chip aus HAProxy-Stats
- Backends: HAProxy-Status-Spalte (UP/DEGRADED/DOWN)
- Settings: Service-Neustart-Karte (haproxy/squid/unbound/chrony/scheduler)
- Settings: Upgrade-Status-Card, Wartungsmodus, Auto-Update, Retention
- Dashboard: Recent-Alerts, Cluster-Health, License-Chip, Onboarding-Hint
- System-Regeln im Firewall als eigener Tab

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

management-ui/src/components/Layout/AppLayout.tsx

@@ -6,6 +6,7 @@ import Sidebar from './Sidebar'
 import Header from './Header'
 import UpdateBanner from '../UpdateBanner'
 import LicenseBanner from '../LicenseBanner'
+import MaintenanceBanner from '../MaintenanceBanner'
 
 // PAGE_TITLES maps the pathname to an i18n nav key. Header reads
 // this to render "where you are". Empty fallback = app.title.
@@ -20,6 +21,7 @@ const PAGE_TITLES: Record<string, string> = {
   '/firewall':      'nav.firewall',
   '/cluster':       'nav.cluster',
   '/logs':          'nav.logs',
+  '/audit':         'nav.audit',
   '/backups':       'nav.backups',
   '/diagnostics':   'nav.diagnostics',
   '/alerts':        'nav.alerts',
@@ -50,6 +52,7 @@ export default function AppLayout() {
 
       <main className="main-content">
         <Header pageTitle={title} onMenuToggle={() => setSidebarOpen(true)} />
+        <MaintenanceBanner />
         <LicenseBanner />
         <UpdateBanner />
         <div className="content-area">