fix: Return 201 instead of 200 on oauth registration

See RFC 7591 at 3.2.1: "The server responds with an HTTP 201 Created status code [...]"

Fixes a failure in matrix-dart-sdk

src/api/client/oauth/register_client.rs

@@ -24,5 +24,5 @@ pub(crate) async fn register_client_route(
 		.await
 		.map_err(|err| (StatusCode::BAD_REQUEST, Json(err)).into_response())?;
 
-	Ok(Json(RegisteredClient { client_id, metadata }).into_response())
+	Ok((StatusCode::CREATED, Json(RegisteredClient { client_id, metadata })).into_response())
 }