Replace IsDown's third-party scraping with a direct reachability check

Replace IsDown's third-party scraping with a direct reachability check

isup.me now serves a Cloudflare bot-challenge page to non-browser clients
(confirmed live), so scraping it can never work again regardless of the
earlier words[0]/words[1] fix. Checked two alternatives from munin:
downforeveryoneorjustme.com is also Cloudflare-gated, and
isitdownrightnow.com, while not gated, only exposes status via a CSS class
in a ping-history table with ambiguous boilerplate text ("is up"/"is down"
appear on every page regardless of actual status) - too fragile to scrape
reliably either.

Instead of depending on any third party's HTML, do the obvious thing: a
DNS lookup to distinguish "doesn't exist" from "unreachable", then a
direct request to the target with a timeout. Simpler, more reliable, and
checks the actual target instead of trusting a third party's cache.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>