fix: workspace delete MissingGreenlet + nginx cache-busting

- Convert WorkspaceHasInstancesError to store plain dicts instead of
  SQLAlchemy ORM objects, preventing lazy-load failures outside async
  session context (MissingGreenlet)
- Update both delete endpoints (top-level and nested) to use exc.instances
  directly since they're already plain dicts
- Add no-cache headers for index.html in nginx.conf so browsers always
  fetch new hashed JS/CSS bundles on deploy

apps/web/nginx.conf

@@ -15,6 +15,12 @@ server {
         try_files $uri $uri/ /index.html;
     }
 
+    # Never cache index.html so browsers always fetch new hashed JS/CSS
+    location = /index.html {
+        add_header Cache-Control "no-cache, no-store, must-revalidate";
+        add_header Pragma "no-cache";
+    }
+
     # Cache static assets
     location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg|woff|woff2)$ {
         expires 1y;