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

src/hooks.server.ts

@@ -6,6 +6,10 @@ import * as userService from '$lib/server/services/userService.js';
 import * as apiTokenService from '$lib/server/services/apiTokenService.js';
 import { extractBearerToken } from '$lib/server/middleware/authenticate.js';
 import { isBoardGuestAccessible } from '$lib/server/middleware/guestAccess.js';
+import { initBackupScheduler } from '$lib/server/jobs/backupScheduler.js';
+
+// Initialize backup scheduler on server startup
+initBackupScheduler();
 
 const PUBLIC_PATHS = ['/login', '/register', '/auth/', '/api/health', '/api/onboarding', '/status'];