audit 4: findings of the two reviews

- Two inbound domains on one upstream could never change its TLS mode,
  each save refused for the other (medium): SetTransportAs changes the
  mode for every domain on the upstream when the one saving may change
  them all; a domain user who may not is refused with the mode, not the
  other tenant's domain name (security F1: the first version named it).
- A deletion's leftover DKIM key is removed by Resync (Reload
  configuration, a restore's first start) for names no domain has.
- The backup's write deadline moves before Check.
- hasUnsavedEdit skips password and file fields and compares a multiple
  select's options; htmx.config.timeout 30 s, so a refresh never answered
  ends and is tried again; a failed refresh's promise is handled.
- An empty filtered Outbound log says no message matches the filters.
- The dummy bcrypt hash is made at start; a DMARC part over 8 MiB is
  refused for its size like the other two limits.
- Health's Configuration box says what Reload writes (design first,
  32dfe4a).
- Docs: CHANGELOG, guide § Inbound, security.md (the shared upstream's
  mode, the profile form's "username taken"), architecture.md (owed to an
  address), the plan's notes corrected.
Tests: the shared upstream (service, handler), the leftover key, all eight
Health boxes ordered, the subject from before migration 0011, the new JS
cases.

Tested on the test server: go vet + go test of 25 packages, make e2e (all
pass, 636 s) on this tree.

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

internal/domain/opendkim.go

@@ -92,6 +92,24 @@ func (o *OpenDKIM) RemoveKey(domainName string) error {
 	return nil
 }
 
+// KeyDomains lists the names that have a key directory.
+func (o *OpenDKIM) KeyDomains() ([]string, error) {
+	entries, err := os.ReadDir(o.keysDir)
+	if os.IsNotExist(err) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("list key dirs: %w", err)
+	}
+	var out []string
+	for _, e := range entries {
+		if e.IsDir() {
+			out = append(out, e.Name())
+		}
+	}
+	return out, nil
+}
+
 // ExportKey returns a domain's DKIM private key as PKCS#1 PEM, for carrying in
 // a domain export so the receiving instance signs with the same key and the
 // DNS TXT record never has to change (architecture.md § Persistence). It