feat(backend): lessons CRUD service and routes

packages/backend/src/index.ts

@@ -1,7 +1,9 @@
 import { createApp } from './app.js';
+import { createDb } from './db/client.js';
 
 const PORT = Number(process.env.PORT ?? 3000);
-const app = createApp();
+const { db } = createDb();
+const app = createApp(db);
 app.listen(PORT, () => {
   console.log(`Backend listening on http://localhost:${PORT}`);
 });