feat: automatische geplante Backups mit Rotation

Phase 1.3 - Automatische Backups:
- Cron-basierter Scheduler (Standard: täglich 2 Uhr)
- Konfigurierbar über .env (Zeitplan, Verzeichnis, Anzahl)
- Automatische Rotation: behält nur letzte N Backups (Standard: 7)
- UI in Settings → Backup: Status-Anzeige und manueller Trigger
- Tests: 7 erfolgreiche Tests für Scheduler-Funktionalität

Neue Umgebungsvariablen:
- BACKUP_ENABLED (Standard: true)
- BACKUP_SCHEDULE (Standard: 0 2 * * *)
- BACKUP_DIR (Standard: ./backups)
- BACKUP_KEEP (Standard: 7)
- TZ (für Zeitzone)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

server/index.js

@@ -17,6 +17,7 @@ import { buildOpenApiSpec } from './openapi.js';
 import * as googleCalendar from './services/google-calendar.js';
 import * as appleCalendar from './services/apple-calendar.js';
 import * as icsSubscription from './services/ics-subscription.js';
+import { startScheduler as startBackupScheduler } from './services/backup-scheduler.js';
 import dashboardRouter from './routes/dashboard.js';
 import tasksRouter from './routes/tasks.js';
 import shoppingRouter from './routes/shopping.js';
@@ -274,6 +275,9 @@ app.listen(PORT, () => {
     setInterval(runSync, SYNC_INTERVAL_MS);
     logSync.info(`Auto-sync active every ${SYNC_INTERVAL_MS / 60_000} minutes.`);
   }, 10_000);
+
+  // Backup-Scheduler starten
+  startBackupScheduler();
 });
 
 export default app;