submit: raise concurrency_limit to 16 so submissions run in parallel

handle_submit blocks for the whole eval (dispatch + poll). With Gradio's
default concurrency of 1, submissions serialized and a single stuck eval froze
the Submit button for everyone. Allow up to 16 concurrent.

Co-authored-by: Cursor <cursoragent@cursor.com>

app.py

@@ -1208,6 +1208,10 @@ pose, and a local self-check), see
             fn=handle_submit,
             inputs=[zip_in],
             outputs=[submit_status],
+            # handle_submit blocks for the whole eval (dispatch + poll), so the
+            # default concurrency of 1 serializes all submissions and a single
+            # stuck eval freezes Submit for everyone. Allow many in parallel.
+            concurrency_limit=16,
         )
 
     with gr.Tab("About"):