feat: add the hot-reload on backend crate

crates/backend/src/main.rs

@@ -1,9 +1,23 @@
+use listenfd::ListenFd;
+use tokio::net::TcpListener;
+
 use backend::get_router;
 
 #[tokio::main]
 async fn main() {
     let app = get_router();
-    let listener = tokio::net::TcpListener::bind("0.0.0.0:8080").await.unwrap();
+
+    let mut listenfd = ListenFd::from_env();
+
+    let listener = match listenfd.take_tcp_listener(0).unwrap() {
+        // if we are given a tcp listener on listen fd 0, we use that one
+        Some(listener) => {
+            listener.set_nonblocking(true).unwrap();
+            TcpListener::from_std(listener).unwrap()
+        }
+        // otherwise fall back to local listening
+        None => TcpListener::bind("0.0.0.0:8080").await.unwrap(),
+    };
 
     println!("Listening on {}", listener.local_addr().unwrap());
     axum::serve(listener, app).await.unwrap();