fix error handler middleware parameter order (#243)

The error handler was using (req, res, next, err) but drachtio-srf
error middleware expects (err, req, res, next). This caused
"res.send is not a function" errors when the handler was invoked.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

app.js

@@ -270,7 +270,7 @@ srf.invite((req, res) => {
   session.connect();
 });
 
-srf.use((req, res, next, err) => {
+srf.use((err, req, res, next) => {
   logger.error(err, 'hit top-level error handler');
   res.send(500);
 });