Serialize builds and add health endpoint to build agent

- Add a non-blocking build lock so only one job runs at a time;
  concurrent upload/checkout requests get a 409 response
- Add /healthz returning 200 when idle and 503 while a build is running
- Ignore .DS_Store artifacts

.gitignore

@@ -2,3 +2,4 @@ known_hosts
 __pycache__/
 *.py[cod]
 *.egg-info/
+.DS_Store

cross_py_builder/build_agent.py

@@ -5,6 +5,7 @@ import signal
 import socket
 import sys
 import tempfile
+import threading
 import time
 
 from flask import Flask, request, jsonify, send_file
@@ -30,6 +31,7 @@ LOCAL_DIR = os.path.dirname(__file__)
 BUILD_DIR = "pybuild-data"
 TMP_DIR = tempfile.gettempdir()
 system_status = {"status": "ready", "running_job": None}
+build_lock = threading.Lock()
 
 def is_windows():
     return platform.system().lower() == "windows"
@@ -127,6 +129,12 @@ def status_page():
     return (f"{APP_NAME} - Build Agent {build_agent_version} - \n"
             f"{system_os()} | {cpu_arch()} | {version} | {hostname} | {ZeroconfServer.get_local_ip()}")
 
+@app.get("/healthz")
+def healthz():
+    ok = system_status['status'] == "ready"
+    return jsonify({"status": "ok" if ok else "busy",
+                    "agent_version": build_agent_version}), 200 if ok else 503
+
 @app.get("/status")
 def status():
     def get_directory_size(directory):
@@ -173,6 +181,9 @@ def checkout_project():
     if not request.is_json or not request.json.get('repo_url'):
         return jsonify({'error': 'Repository URL is required'}), 400
 
+    if not build_lock.acquire(blocking=False):
+        return jsonify({'error': f"Another build is already running: {system_status['running_job']}"}), 409
+
     repo_url = request.json['repo_url']
     print(f"\n========== Checking Out Git Project ==========")
 
@@ -194,6 +205,8 @@ def checkout_project():
         if isinstance(e, subprocess.CalledProcessError):
             return jsonify({'error': 'Failed to clone repository'}), 500
         return jsonify({'error': f"Uncaught error processing checkout: {e}"}), 500
+    finally:
+        build_lock.release()
 
 def safe_extract(zip_ref, dest_dir):
     """Extract a zipfile, rejecting members that would escape dest_dir."""
@@ -206,6 +219,9 @@ def safe_extract(zip_ref, dest_dir):
 
 @app.route('/upload', methods=['POST'])
 def upload_project():
+    if not build_lock.acquire(blocking=False):
+        return jsonify({'error': f"Another build is already running: {system_status['running_job']}"}), 409
+
     try:
         start_time = datetime.datetime.now()
         if 'file' not in request.files:
@@ -234,6 +250,8 @@ def upload_project():
         system_status['status'] = "ready"
         shutil.rmtree(working_dir, ignore_errors=True)
         return jsonify({"error": f"Uncaught error processing job: {e}"}), 500
+    finally:
+        build_lock.release()
 
 def install_and_build(project_path, job_id, start_time):