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>

internal/database/migrations/0023_domains_settings.sql

@@ -0,0 +1,39 @@
+-- +goose Up
+-- +goose StatementBegin
+
+-- Domain-Settings die HAProxy pro Domain rendern kann. Bisher
+-- hatten Domains nur ein binäres hsts_enabled — jetzt ist HSTS
+-- granular (max-age, subdomains, preload) plus Maintenance-Mode
+-- und Canonical-www-Redirect.
+
+ALTER TABLE domains
+    ADD COLUMN IF NOT EXISTS hsts_max_age        INTEGER NOT NULL DEFAULT 31536000,
+    ADD COLUMN IF NOT EXISTS hsts_subdomains     BOOLEAN NOT NULL DEFAULT FALSE,
+    ADD COLUMN IF NOT EXISTS hsts_preload        BOOLEAN NOT NULL DEFAULT FALSE,
+    ADD COLUMN IF NOT EXISTS maintenance_mode    BOOLEAN NOT NULL DEFAULT FALSE,
+    ADD COLUMN IF NOT EXISTS maintenance_message TEXT,
+    ADD COLUMN IF NOT EXISTS www_redirect        TEXT NOT NULL DEFAULT '';
+
+-- www_redirect-Werte:
+--   ''        — kein Redirect (Default; Domain wird direkt geserved)
+--   'to-naked'— www.foo.com → foo.com (canonical naked-domain)
+--   'to-www'  — foo.com → www.foo.com (canonical www)
+ALTER TABLE domains
+    DROP CONSTRAINT IF EXISTS domains_www_redirect_check;
+ALTER TABLE domains
+    ADD CONSTRAINT domains_www_redirect_check
+        CHECK (www_redirect IN ('', 'to-naked', 'to-www'));
+
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+ALTER TABLE domains DROP CONSTRAINT IF EXISTS domains_www_redirect_check;
+ALTER TABLE domains
+    DROP COLUMN IF EXISTS www_redirect,
+    DROP COLUMN IF EXISTS maintenance_message,
+    DROP COLUMN IF EXISTS maintenance_mode,
+    DROP COLUMN IF EXISTS hsts_preload,
+    DROP COLUMN IF EXISTS hsts_subdomains,
+    DROP COLUMN IF EXISTS hsts_max_age;
+-- +goose StatementEnd