Reusable smoke test for the camoufox HTTP service. Run from the deploy host:
docker exec -i tradein-backend sh < tradein-mvp/scripts/browser-smoke.sh
Reports health, byte count, preloadedState presence, firewall-block detection
+ first 200 bytes of the response. Avoids long one-line curl commands that get
mangled on terminal paste.
Refs #905, #883