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/api/src/api/workspaces.py

@@ -91,7 +91,7 @@ async def delete_workspace_top_level(
             status_code=409,
             detail={
                 "message": "Workspace has running tool instances",
-                "instances": [{"id": str(i.id), "name": i.name} for i in exc.instances],
+                "instances": exc.instances,
             },
         ) from exc
     except Exception as exc:
@@ -358,7 +358,7 @@ async def delete_workspace(
             status_code=409,
             detail={
                 "message": "Workspace has running tool instances",
-                "instances": [{"id": str(i.id), "name": i.name} for i in exc.instances],
+                "instances": exc.instances,
             },
         ) from exc
     except Exception as exc: