Fix backend web UI 404 error with absolute path

Applied the same web_ui.html path fix to backend/api_server.py that
was previously applied to the root api_server.py. The backend version
needs to reference the parent directory since web_ui.html is located
at /opt/ai_qc/web_ui.html while api_server.py is in /opt/ai_qc/backend/.

This fixes the 404 error when running via Waitress production server.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/api_server.py

@@ -1303,7 +1303,10 @@ def load_qc_apps():
 def serve_web_ui():
     """Serve the web UI"""
     try:
-        with open('web_ui.html', 'r') as f:
+        # Use absolute path to web_ui.html (located in parent directory)
+        base_dir = os.path.dirname(os.path.abspath(__file__))
+        web_ui_path = os.path.join(os.path.dirname(base_dir), 'web_ui.html')
+        with open(web_ui_path, 'r') as f:
             html_content = f.read()
         return Response(html_content, mimetype='text/html')
     except FileNotFoundError: