Refactor workflow state management in site building; enhance error handling and field validation in models and serializers. Remove obsolete workflow components from frontend and adjust API response structure for clarity.

frontend/src/components/sites/SiteProgressWidget.tsx

@@ -293,16 +293,6 @@ export default function SiteProgressWidget({ blueprintId, siteId }: SiteProgress
         </div>
       </div>
 
-      {/* Deep Link to Blueprint */}
-      <div className="mt-6 pt-6 border-t border-gray-200 dark:border-gray-700">
-        <button
-          onClick={() => navigate(`/sites/builder/workflow/${blueprintId}`)}
-          className="w-full px-4 py-2 bg-blue-500 hover:bg-blue-600 text-white rounded-lg text-sm font-medium transition-colors focus:outline-none focus:ring-2 focus:ring-blue-500 focus:ring-offset-2"
-          aria-label="Continue site builder workflow"
-        >
-          Continue Site Builder Workflow →
-        </button>
-      </div>
       
       {/* Error banner if data loaded but has errors */}
       {error && progress && (