fix: update CONFIRM_API_URL in .env.example to NoviSteps organization crawler

Replace the placeholder URL with the actual NoviSteps organization server
(atcoder-novisteps-crawler.vercel.app/api) as requested by the team.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

Author: river0525

.env.example

@@ -1,3 +1,3 @@
 # AtCoder affiliation confirmation API endpoint
 # Set this to the actual endpoint URL in your .env file (do not commit real URLs here)
-CONFIRM_API_URL=https://your-confirm-api-endpoint.example.com/confirm
+CONFIRM_API_URL=https://atcoder-novisteps-crawler.vercel.app/api