feat(backup): replace JSON import/export with SQLite database backup system

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

plans/database-backup/PLAN.md

@@ -0,0 +1,39 @@
+# Feature: Database Backup & Restore
+
+**Branch:** `feature/database-backup`
+**Base branch:** `master`
+**Created:** 2026-04-02
+**Status:** 🟡 In Progress
+**Strategy:** Big Bang
+**Mode:** Automated
+**Execution:** Direct
+
+## Summary
+Replace JSON import/export with SQLite file-copy backup system. Support manual on-demand backups, optional periodic scheduling via node-cron, configurable retention (max backup count), download capability, and full database restore.
+
+## Build & Test Commands
+- **Build:** `npm run build`
+- **Test:** `npm run test`
+- **Lint:** `npm run lint`
+
+## Phases
+
+- [ ] Phase 1: Backup Service & API Endpoints [domain: backend] → [subplan](./phase-1-backup-service.md)
+- [ ] Phase 2: Periodic Backup Scheduler [domain: backend] → [subplan](./phase-2-backup-scheduler.md)
+- [ ] Phase 3: Frontend BackupPanel [domain: frontend] → [subplan](./phase-3-backup-panel.md)
+- [ ] Phase 4: Cleanup Import/Export [domain: fullstack] → [subplan](./phase-4-cleanup.md)
+
+## Phase Progress Log
+
+| Phase | Domain | Status | Review | Build | Committed |
+|-------|--------|--------|--------|-------|-----------|
+| Phase 1: Backup Service & API | backend | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
+| Phase 2: Backup Scheduler | backend | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
+| Phase 3: BackupPanel UI | frontend | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
+| Phase 4: Cleanup | fullstack | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
+
+## Final Review
+- [ ] Comprehensive code review
+- [ ] Full build passes
+- [ ] Full test suite passes
+- [ ] Merged to `master`