Stream live worker progress via async build dispatch

The agent's /upload and /checkout_git were synchronous, blocking until a
build finished, so the controller could not relay live per-step progress.
Add ?async=1 support: the agent saves/clones the source, starts the build in
a background thread (holding build_lock), and returns 202 + {"id": ...}
immediately. The controller dispatches async, then polls /progress/<job_id>,
appending each step/log line to the job log and SSE stream, and treats the
build as done when progress clears (404) and the worker reports ready.

cross_py_builder/build_agent.py

@@ -220,6 +220,7 @@ def checkout_project():
     if not build_lock.acquire(blocking=False):
         return jsonify({'error': f"Another build is already running: {system_status['running_job']}"}), 409
 
+    async_mode = _is_async_request()
     repo_url = request.json['repo_url']
     print(f"\n========== Checking Out Git Project ==========")
 
@@ -233,6 +234,9 @@ def checkout_project():
         system_status['status'] = "cloning_repo"
         subprocess.check_call(['git', 'clone', repo_url, repo_dir])
         system_status['status'] = "ready"
+        if async_mode:
+            _run_build_background(job_id, repo_dir, start_time)
+            return jsonify({"id": job_id, "async": True, "message": "Build started"}), 202
         return install_and_build(repo_dir, job_id, start_time)
     except Exception as e:
         print(f"Error processing checkout: {e}")
@@ -244,7 +248,26 @@ def checkout_project():
             return jsonify({'error': 'Failed to clone repository'}), 500
         return jsonify({'error': f"Uncaught error processing checkout: {e}"}), 500
     finally:
-        build_lock.release()
+        if not async_mode:
+            build_lock.release()
+
+def _run_build_background(job_id, project_path, start_time):
+    """Run install_and_build in a background thread. Assumes build_lock is held;
+    the thread releases it when the build finishes."""
+    def runner():
+        try:
+            install_and_build(project_path, job_id, start_time)
+        except Exception as e:
+            print(f"Background build failed for {job_id}: {e}")
+            _clear_progress(job_id)
+        finally:
+            build_lock.release()
+    threading.Thread(target=runner, name=f"build-{job_id}", daemon=True).start()
+
+
+def _is_async_request():
+    return request.args.get("async") in ("1", "true", "yes", "on")
+
 
 def safe_extract(zip_ref, dest_dir):
     """Extract a zipfile, rejecting members that would escape dest_dir."""
@@ -260,6 +283,8 @@ def upload_project():
     if not build_lock.acquire(blocking=False):
         return jsonify({'error': f"Another build is already running: {system_status['running_job']}"}), 409
 
+    async_mode = _is_async_request()
+    working_dir = None
     try:
         start_time = datetime.datetime.now()
         if 'file' not in request.files:
@@ -283,15 +308,20 @@ def upload_project():
             print(f"Extracting uploaded project zip...")
             safe_extract(zip_ref, working_dir)
 
+        if async_mode:
+            _run_build_background(job_id, working_dir, start_time)
+            return jsonify({"id": job_id, "async": True, "message": "Build started"}), 202
+
         return install_and_build(working_dir, job_id, start_time)
     except Exception as e:
         print(f"Uncaught error processing job: {e}")
         system_status['status'] = "ready"
         _clear_progress(job_id)
-        shutil.rmtree(working_dir, ignore_errors=True)
+        shutil.rmtree(working_dir, ignore_errors=True) if working_dir else None
         return jsonify({"error": f"Uncaught error processing job: {e}"}), 500
     finally:
-        build_lock.release()
+        if not async_mode:
+            build_lock.release()
 
 def install_and_build(project_path, job_id, start_time):
     _set_step(job_id, "starting", project_path=project_path)