fix(npdl): Add Last-Modified header on response

This allows Express to return `304 Not Modified` when the console
already has cached the requested BOSS file.

src/services/npdl.ts

@@ -36,6 +36,7 @@ npdl.get([
 		return;
 	}
 
+	response.setHeader('Last-Modified', new Date(Number(file.updated)).toUTCString());
 	readStream.pipe(response);
 });
 
@@ -43,4 +44,4 @@ const router = express.Router();
 
 router.use(subdomain('npdl.cdn', npdl));
 
-export default router;
+export default router;