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 <noreply@anthropic.com>

frontend/src/components/admin/AdminSidebar.tsx

@@ -9,7 +9,8 @@ import {
   Settings,
   X,
   Palette,
-  FileText
+  FileText,
+  HardDrive
 } from 'lucide-react';
 import { useQuery } from '@tanstack/react-query';
 import { useTranslation } from 'react-i18next';
@@ -35,6 +36,7 @@ const navigation: NavItem[] = [
   { nameKey: 'navigation.emailSettings', href: '/admin/email', icon: Mail },
   { nameKey: 'navigation.branding', href: '/admin/branding', icon: Palette },
   { nameKey: 'navigation.settings', href: '/admin/settings', icon: Settings },
+  { nameKey: 'navigation.backup', href: '/admin/backup', icon: HardDrive },
   { nameKey: 'navigation.cmsPages', href: '/admin/cms', icon: FileText },
 ];