Replace the JSON-based import/export with a proper backup system that copies
the SQLite database file directly. Supports manual on-demand backups, periodic
scheduled backups via node-cron, configurable retention, file download, and
full database restore.
- Add backupService with VACUUM INTO for safe DB copies
- Add backupScheduler following healthcheckScheduler pattern
- Add 6 admin API endpoints (create, list, download, restore, delete, schedule)
- Add BackupPanel UI with backup table, confirmation dialogs, schedule config
- Add backup fields to SystemSettings schema
- Remove old ImportExportPanel, exportService, importService, and related code
