feat: setup a backend server with axum

crates/backend/src/lib.rs

@@ -0,0 +1,12 @@
+use axum::http::{StatusCode, Uri};
+use axum::{routing::get, Router};
+
+pub fn get_router() -> Router {
+    Router::new()
+        .route("/", get(|| async { "Hello, world!" }))
+        .fallback(fallback)
+}
+
+async fn fallback(uri: Uri) -> (StatusCode, String) {
+    (StatusCode::NOT_FOUND, format!("No route for {uri}"))
+}