Adds rescan_dead flag that causes _filter_unvalidated to treat
previously-dead domains as needing a fresh check. Useful after
fixing the http/https detection bug.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Many modern servers refuse HTTP connections entirely. The validator was
only trying http://, causing HTTPS-only sites to be wrongly marked dead.
Now falls back to https:// on ConnectError. Also increased timeouts slightly.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>