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/app/service.go

@@ -17,6 +17,15 @@ import (
 // *postfix.Postfix satisfies it; tests substitute a fake.
 type SenderMaps interface {
 	RebuildSenderLoginMaps(bindings []postfix.Binding) error
+	// Reload has Postfix re-read its configuration whatever changed.
+	Reload() error
+}
+
+// ReloadPostfix has Postfix re-read its configuration. A rebuild that
+// changes no file asks no reload; Health's Reload configuration ends with
+// this one, so that it re-reads whatever is on disk.
+func (s *Service) ReloadPostfix() error {
+	return s.pf.Reload()
 }
 
 // Service coordinates application state across SQLite, sasldb2 and the Postfix

internal/app/service_test.go

@@ -20,6 +20,8 @@ type fakeMaps struct {
 	failNext bool
 }
 
+func (f *fakeMaps) Reload() error { return nil }
+
 func (f *fakeMaps) RebuildSenderLoginMaps(b []postfix.Binding) error {
 	f.calls++
 	if f.failNext {
@@ -514,6 +516,8 @@ type overlapMaps struct {
 	inside, most atomic.Int32
 }
 
+func (o *overlapMaps) Reload() error { return nil }
+
 func (o *overlapMaps) RebuildSenderLoginMaps([]postfix.Binding) error {
 	n := o.inside.Add(1)
 	for {