feat(backend): bootstrap express app with error handling

packages/backend/src/index.ts

@@ -0,0 +1,7 @@
+import { createApp } from './app.js';
+
+const PORT = Number(process.env.PORT ?? 3000);
+const app = createApp();
+app.listen(PORT, () => {
+  console.log(`Backend listening on http://localhost:${PORT}`);
+});