feat(alerts): Health-Alarme via Webhook + Email-SMTP

Sidebar → System → Alarme.

Migration 0021: alert_channels (kind=webhook|email, target, settings,
active) + alert_events (kind, severity=info/warning/error/critical,
subject, message, sent_to JSONB).

internal/services/alerts/:
  - Fire(kind, severity, subject, message) — broadcastet an alle
    aktiven Channels + persistiert Event mit per-Channel-Result
    (ok/error) in sent_to.
  - Webhook-Sender: POST JSON {kind, severity, subject, message,
    content, text, fired_at}. Slack/Discord/Teams akzeptieren das
    out-of-the-box ohne Adapter (content + text-Felder gleichzeitig).
  - Email-Sender: net/smtp + STARTTLS optional. Settings (smtp_host,
    smtp_port, username/password, from, use_tls) liegen in
    channel.settings JSONB.

internal/handlers/alerts.go: CRUD + POST /alerts/test + GET
/alerts/events (history).

Scheduler-Trigger:
  - cert.expiring  — TLS-Cert <14 Tage Restzeit (12h-dedupe pro cert)
                     severity warning, <3 Tage → error
  - cert.renew_failed       — Renewer-Cycle hat fails
  - cert.renewer.run_failed — Renewer-Cycle abgebrochen
  - backup.failed  — Scheduled Backup error
  - license.invalid — License-Server liefert valid=false

In-process Dedupe (12h TTL, map[key]time.Time) verhindert dass
identische Alerts in Schleifen feuern.

UI (pages/Alerts): Tabs Channels (CRUD-Tabelle, Add-Modal mit
conditional-Email-Fields) + History (200 letzte Events mit
severity-Tag + per-Channel-Delivery-Status). Header-Button
„Test-Alert" feuert einen Test-Event in alle aktiven Channels.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

management-ui/src/App.tsx

@@ -29,6 +29,7 @@ const FirewallLivePage = lazy(() => import('./pages/FirewallLive'))
 const LogsPage = lazy(() => import('./pages/Logs'))
 const BackupsPage = lazy(() => import('./pages/Backups'))
 const DiagnosticsPage = lazy(() => import('./pages/Diagnostics'))
+const AlertsPage = lazy(() => import('./pages/Alerts'))
 const LicensePage = lazy(() => import('./pages/License'))
 const SettingsPage = lazy(() => import('./pages/Settings'))
 
@@ -117,6 +118,7 @@ export default function App() {
                   <Route path="/logs" element={<LogsPage />} />
                   <Route path="/backups" element={<BackupsPage />} />
                   <Route path="/diagnostics" element={<DiagnosticsPage />} />
+                  <Route path="/alerts" element={<AlertsPage />} />
                   <Route path="/license" element={<LicensePage />} />
                   <Route path="/settings" element={<SettingsPage />} />
                 </Route>