Summary

• Pins Node engine to 20.x in package.json ahead of the Heroku stack upgrade from heroku-22 to heroku-24 (already set via heroku stack:set heroku-24).
• Heroku-24 defaults differ from heroku-22, so an explicit pin avoids surprises on the first deploy.
• Follow-up to bump to Node 22 (active LTS) tracked in Bump Node from 20 to 22 (LTS) #621.

Test plan

• Merge, deploy, verify build succeeds on heroku-24 with Node 20.x