fix: make healthcheck ping quota-free api docs page

[vlnst]

/healthz always returns 503: https://stackoverflow.com redirects to a Cloudflare challenge (403), even when the app works fine with SCRAPER=api.

With SCRAPER=api, the healthcheck now pings https://api.stackexchange.com/docs instead, since that's what the app depends on. /docs doesn't use API quota, unlike /2.3/* methods. A check every 60s would use up the 300/day anonymous quota on its own. The HTML scraper keeps pinging stackoverflow.com, since it depends on that. The unused stackexchange.com check is removed.

The Docker healthcheck binary ignored the status code, so the container showed as healthy even when /healthz returned 503. It now exits 1 on anything but 200.

Tested: /healthz returns 200 with SCRAPER=api; the healthcheck binary exits 0 there, and 1 when /healthz returns 503 or the app is down.