Release v1.0.2: Database Backup Feature

🆕 New Features:
- Automatic database backups every N days (configurable via backup_days)
- Manual backup button for admins (💾 Бэкап БД)
- Backup scheduler with initial backup 1 minute after start
- Backup files include timestamp and size metadata

🔧 Configuration:
- Added panel.backup_days parameter (0 = disabled, default: 7)

🛠️ Technical Changes:
- Implemented GetDatabaseBackup() API method
- Added backupScheduler() goroutine with graceful shutdown
- Added handleBackupRequest() for manual backups
- Custom namedBytesReader for in-memory file sending
- Updated admin keyboard layout

📝 Documentation:
- Updated README with backup features
- Updated config.yaml.example

🔒 Security:
- Admin-only access with permission checks
- Proper error handling and logging