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/components/Layout/Sidebar.tsx

@@ -2,6 +2,7 @@ import { Link, useLocation } from 'react-router-dom'
 import type { ReactNode } from 'react'
 import {
   ApartmentOutlined,
+  BellOutlined,
   ClockCircleOutlined,
   CloudServerOutlined,
   ClusterOutlined,
@@ -76,6 +77,7 @@ const NAV: NavSection[] = [
       { path: '/cluster',     labelKey: 'nav.cluster',     icon: <ApartmentOutlined /> },
       { path: '/logs',        labelKey: 'nav.logs',        icon: <FileSearchOutlined /> },
       { path: '/diagnostics', labelKey: 'nav.diagnostics', icon: <ToolOutlined /> },
+      { path: '/alerts',   labelKey: 'nav.alerts',   icon: <BellOutlined /> },
       { path: '/backups',  labelKey: 'nav.backups',  icon: <DatabaseOutlined /> },
       { path: '/license',  labelKey: 'nav.license',  icon: <CrownOutlined /> },
       { path: '/settings', labelKey: 'nav.settings', icon: <SettingOutlined /> },
@@ -83,7 +85,7 @@ const NAV: NavSection[] = [
   },
 ]
 
-const VERSION = '1.0.73'
+const VERSION = '1.0.74'
 
 // Sidebar-Pattern 1:1 aus netcell-webpanel (enconf) übernommen:
 // - <nav> als root, dunkler Gradient + Teal/Blue-Accent