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/routes/admin/settings/+page.svelte

@@ -2,7 +2,7 @@
 	import { t } from 'svelte-i18n';
 	import type { PageData } from './$types.js';
 	import SettingsForm from '$lib/components/admin/SettingsForm.svelte';
-	import ImportExportPanel from '$lib/components/admin/ImportExportPanel.svelte';
+	import BackupPanel from '$lib/components/admin/BackupPanel.svelte';
 	import DiscoveryPanel from '$lib/components/admin/DiscoveryPanel.svelte';
 
 	let { data }: { data: PageData } = $props();
@@ -25,5 +25,5 @@
 
 	<DiscoveryPanel bind:dockerSocketPath bind:traefikApiUrl />
 
-	<ImportExportPanel />
+	<BackupPanel />
 </div>