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_apps.go

@@ -273,10 +273,26 @@ func (h *Handlers) HandleDomainDNSRecheck(w http.ResponseWriter, r *http.Request
 		http.Error(w, "internal error", http.StatusInternalServerError)
 		return
 	}
+	if !h.allowRecheck(r) {
+		http.Redirect(w, r, fmt.Sprintf("/outbound/domains/%d", d.ID), http.StatusSeeOther)
+		return
+	}
 	h.domainDNS(d, record, d.DMARCRua, true)
 	http.Redirect(w, r, fmt.Sprintf("/outbound/domains/%d?rechecked=1", d.ID), http.StatusSeeOther)
 }
 
+// allowRecheck is whether the signed-in user may force the DNS checks now
+// (auth.AllowDNSRecheck). Past the limit the page shows the cached result,
+// without saying it re-checked; the log says why.
+func (h *Handlers) allowRecheck(r *http.Request) bool {
+	p, ok := h.principal(r)
+	if !ok || h.auth == nil || h.auth.AllowDNSRecheck(p.ID) {
+		return true
+	}
+	logf("panel: user %d: DNS re-checks asked for too often; the cached result is shown", p.ID)
+	return false
+}
+
 // limitChoice names which of the form's three limit choices an application has:
 // the domain limit (none of its own), manual, or auto.
 func limitChoice(rl store.RateLimit, configured bool) string {