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/api/admin/backups/[filename]/download/+server.ts

@@ -0,0 +1,33 @@
+import { json } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { requireAdmin } from '$lib/server/middleware/authorize.js';
+import { getBackupFilePath } from '$lib/server/services/backupService.js';
+import { error } from '$lib/server/utils/response.js';
+import fs from 'node:fs';
+import path from 'node:path';
+import { Readable } from 'node:stream';
+
+/**
+ * GET /api/admin/backups/:filename/download — Download a backup file (streamed).
+ */
+export const GET: RequestHandler = async (event) => {
+	requireAdmin(event);
+	const { filename } = event.params;
+
+	const filePath = getBackupFilePath(filename);
+	if (!filePath) {
+		return json(error('Backup not found'), { status: 404 });
+	}
+
+	const stats = fs.statSync(filePath);
+	const stream = fs.createReadStream(filePath);
+
+	return new Response(Readable.toWeb(stream) as ReadableStream, {
+		status: 200,
+		headers: {
+			'Content-Type': 'application/octet-stream',
+			'Content-Disposition': `attachment; filename="${path.basename(filePath)}"`,
+			'Content-Length': String(stats.size)
+		}
+	});
+};