Confirm changes automatically from the client that made them

A change made over the network answers with a one-time confirm_token
and closes its connection (HTTP/2: a GOAWAY). The web UI and hc send the
token straight back to /confirm, which accepts it only for the newest
change, from the same user and API token, the same address, and over a
new connection: proof that the client can still reach the API through
the new ruleset (an established connection would get through anyway). A
change that cuts its client off can't be confirmed so, and rolls back by
itself. The setting manual_confirm (hc settings set --auto-confirm off)
and hc --no-confirm leave confirming to a person. Changes through the
local socket still need no confirmation. e2e: 136 checks, among them a
lockout change rolled back without anyone confirming.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/h_changesets.go

@@ -368,6 +368,7 @@ func (s *Server) applyChangeset(w http.ResponseWriter, r *http.Request, p *auth.
 	s.eng.Audit(r.Context(), p, "changeset.apply", fmt.Sprintf("changeset %d", id), "success", fmt.Sprintf("history %d", res.HistoryID))
 	s.afterChange(r.Context(), p, res)
 	out := s.mutation(res, nil)
+	out.ConfirmToken = s.issueCheck(w, r, p, res)
 	out.Changeset = cs
 	writeJSON(w, http.StatusOK, out)
 	return nil