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/CONTEXT.md

@@ -0,0 +1,40 @@
+# Feature Context: Database Backup & Restore
+
+## Configuration
+- **Development mode:** Automated
+- **Execution mode:** Direct
+- **Strategy:** Big Bang
+- **Build:** `npm run build`
+- **Test:** `npm run test`
+- **Lint:** `npm run lint`
+- **Dev server:** `npm run dev` (port: 5173)
+
+## Current State
+Starting implementation. Existing import/export code is still in place — will be removed in Phase 4.
+
+## Tech Stack
+- SvelteKit 5 + TypeScript
+- Prisma ORM with SQLite (`data/launcher.db`)
+- node-cron (already a dependency, used by healthcheckScheduler)
+- Zod validation
+- Tailwind CSS + bits-ui components
+- svelte-i18n for localization
+- lucide-svelte for icons
+
+## Key Patterns from Codebase
+- Services in `src/lib/server/services/` — pure functions, Prisma transactions
+- API routes follow SvelteKit conventions with admin auth guards
+- Scheduler pattern: `src/lib/server/jobs/healthcheckScheduler.ts` — cron start/stop, wired in hooks.server.ts
+- Admin components in `src/lib/components/admin/`
+- Audit logging via `auditLogService.ts`
+
+## Cross-Phase Dependencies
+- Phase 2 depends on Phase 1 (backupService)
+- Phase 3 depends on Phases 1+2 (API endpoints)
+- Phase 4 is independent cleanup
+
+## Implementation Notes
+- Using `VACUUM INTO` for safe backup creation (no locking)
+- Backups stored in `data/backups/` directory
+- Restore requires Prisma disconnect → file swap → reconnect
+- SystemSettings gets new fields: backupEnabled, backupCronExpression, backupMaxCount