Add database backup/restore feature with admin UI

- Backup service using VACUUM INTO for WAL-safe atomic snapshots
- Server-side storage in DATA_DIR/backups with download/upload API routes
- Restore with integrity validation and automatic pre-restore safety backup
- Scheduled daily backups via cron with configurable retention
- Admin-only settings UI consolidated under single Server section
- Clean up account section sign-out button, fix switch sub-pixel rendering

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/db/client.ts

@@ -46,6 +46,9 @@ export const db = new Proxy({} as ReturnType<typeof drizzle>, {
   },
 });
 
+/** Close the current connection, and clear singletons so the Proxy re-initializes on next access. */
 export function closeDatabase() {
   globalForDb._client?.close();
+  globalForDb._client = undefined;
+  globalForDb._db = undefined;
 }