docs, hub, installer: say Server + Desktop makes the home IP public

Server + Desktop publishes services under the operator's own domain, and
the DNS record for that domain points at the home connection, so anyone
can look up the home IP address. None of the places that offer Server +
Desktop said so.

- README: new section "Server + Desktop and your home IP address" (what
  becomes public, what does not, the alternatives, and what happens
  technically), plus a note on the role table and in the security
  overview.
- SECURITY.md: a matching section, the consequence noted next to "Public
  web services exposed only when enabled by the operator", and the
  supported versions row no longer pins 1.0.x.
- ISO installer: the Server + Desktop role card ends with the warning.
- Hub: the domain setup text (onboarding, feature setup and domain
  reconfiguration share renderDomainNeedsHtml) and the upgrade dialog
  carry the same notice.
- Add tests/test_exposure_guards.py. It fails if one of these places
  loses the notice or the README anchor stops resolving.

app/sovran_systemsos_web/static/js/domain-prereqs.js

@@ -43,6 +43,11 @@ function renderDomainNeedsHtml(opts) {
       + '<a href="https://njal.la" target="_blank" rel="noopener noreferrer" style="color:var(--accent-color);">Njal.la</a>'
       + " and connecting your services. Just follow the steps below.</p>";
   }
+  // Every variant above ends with a domain that points at the user's home
+  // connection. Say what that publishes (README: "Server + Desktop and your
+  // home IP address").
+  html += "<p>⚠️ <strong>Heads-up:</strong> your domain points at your home internet connection, "
+    + "so anyone can look up your home IP address. Domain privacy does not hide it.</p>";
   return html;
 }