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>

.env.example

@@ -33,6 +33,13 @@ APPLE_APP_SPECIFIC_PASSWORD=
 # Calendar sync interval in minutes (default: 15)
 SYNC_INTERVAL_MINUTES=15
 
+# Automatic Backups
+# BACKUP_ENABLED=true                  # Enable/disable automated backups (default: true)
+# BACKUP_SCHEDULE=0 2 * * *            # Cron schedule (default: 2 AM daily)
+# BACKUP_DIR=./backups                 # Backup directory (default: ./backups)
+# BACKUP_KEEP=7                        # Number of backups to keep (default: 7)
+# TZ=Europe/Berlin                     # Timezone for scheduled backups (default: UTC)
+
 # Security
 RATE_LIMIT_WINDOW_MS=60000
 RATE_LIMIT_MAX_ATTEMPTS=5