hsts grading was gated on the scheme of the originally requested url,
but the client follows redirects, so an http target that redirects to
https skipped the hsts check entirely and dropped a high-severity
finding. decide the scheme from the final response request url instead,
falling back to the requested url only when no response request is set.
adds a -sh/--security-headers scan that flags missing or weak response
headers (hsts, csp, x-frame-options, x-content-type-options,
referrer-policy, permissions-policy, coop) and headers that leak server
internals (server, x-powered-by, ...). hsts is only graded over https
where it actually applies. wired into App.Run and the module results.
