postfix: no reload for a map that did not change; DNS re-checks limited (audit W6/M9)

Security audit 2026-10-17, findings W6/M9 (low), stage 4. Every
application, upstream, recipient or report-address save reloaded Postfix
through supervisord, and POST .../dns-recheck bypassed the five-minute
cache for up to ~14 resolver queries - both at whatever rate an
authenticated user pressed.

- writeChanged: each generated map is written only when its content
  differs from the file on disk, and Postfix is reloaded only when one was
  (sender map, inbound maps, DMARC maps). Health's Reload configuration
  ends with an explicit reload (app.Service.ReloadPostfix, SenderMaps
  gains Reload), so it still re-reads whatever is on disk. No debounce:
  with unchanged saves gone, a burst is real changes Postfix must see.
- auth.AllowDNSRecheck: twenty forced re-checks in 15 minutes per user;
  past that the outbound, inbound and settings re-check show the cached
  result without the "re-checked" flash, and the log says why.

Tests: TestARebuildThatChangesNothingDoesNotReload,
TestDNSRechecksArePerUser; the fakes gain Reload. Docs: guide § Overview
and Health; CHANGELOG; plan.

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

internal/web/handlers/handlers_domains.go

@@ -206,6 +206,9 @@ func (h *Handlers) HandleReload(w http.ResponseWriter, r *http.Request) {
 	if h.dmarc != nil && h.dmarc.Enabled() {
 		steps = append(steps, step{"the DMARC report maps", h.dmarc.Resync})
 	}
+	// A rebuild that changed nothing asked no reload (security audit
+	// 2026-10-17, W6): this button re-reads whatever is on disk.
+	steps = append(steps, step{"Postfix's reload", h.apps.ReloadPostfix})
 	for _, s := range steps {
 		if err := s.run(); err != nil {
 			logf("panel: manual reload (%s): %v", s.name, err)