feat: implement comprehensive backup and restore system with S3 support

- Add S3/MinIO storage adapter with multipart upload support
- Implement database backup service for SQLite and PostgreSQL
- Create backup manifest generator for tracking backup contents
- Enhance backup service with S3 integration and incremental backups
- Add restore service with safety measures and rollback capability
- Create comprehensive test suite for all backup functionality
- Add admin API endpoints for backup/restore management
- Implement frontend UI with dashboard, configuration, and restore wizard
- Add roadmap section to README with implemented backup feature

This implementation provides:
- Multiple backup destinations (local, rsync, S3/MinIO)
- Intelligent change detection to minimize backup frequency
- Full database backups with compression
- Manifest-based restore with integrity validation
- Pre-restore safety backups with rollback
- Comprehensive error handling and monitoring
- User-friendly admin interface

🤖 Generated with Claude Code

Co-Authored-By: Claude <[email protected]>

frontend/src/i18n/locales/de.json

@@ -61,6 +61,7 @@
     "branding": "Branding",
     "analytics": "Analytik",
     "emailSettings": "E-Mail-Einstellungen",
+    "backup": "Backup & Wiederherstellung",
     "cmsPages": "CMS-Seiten"
   },
   "archives": {

frontend/src/i18n/locales/en.json

@@ -61,6 +61,7 @@
     "branding": "Branding",
     "analytics": "Analytics",
     "emailSettings": "Email Settings",
+    "backup": "Backup & Restore",
     "cmsPages": "CMS Pages"
   },
   "archives": {