panel-notifications: guide, architecture, security; refusals looked for every minute

guide.md gains Notifications: the four things, when each is said and said
again, who is told, the two switches, and the two DNS records of the server's
own name that make the mail arrive. architecture.md: the notifier as the
fifth role, its readings, state, recipients and sending; the server's own key
in OpenDKIM's tables; non_smtpd_milters. security.md: the panel submits mail
of its own - how, to whom, and what text from outside can reach a body.

That text - a queued message's sender, a DNS finding - is reduced to
printable ASCII and cut before it is printed. Refused mail is looked for
every minute instead of every five: it is one indexed query, and a refusal is
mail not being sent now. The image's note on the panel's postdrop membership
no longer calls it read-side only.

go test ./internal/notify passes.

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

internal/notify/text.go

@@ -35,6 +35,24 @@ func compose(hostname, subject string, paragraphs ...string) message {
 
 func stamp(t time.Time) string { return t.UTC().Format("2006-01-02 15:04 UTC") }
 
+// plain makes text that came from outside fit for a message body: a queued
+// message's sender is whatever its sender wrote, and a DNS finding may quote a
+// name. What is not printable ASCII becomes "?", and it is cut to max bytes -
+// a notification is read by a person, and nothing in it is theirs to act on
+// but the panel's own words.
+func plain(s string, max int) string {
+	if len(s) > max {
+		s = s[:max]
+	}
+	b := []byte(s)
+	for i, c := range b {
+		if c < 0x20 || c > 0x7e {
+			b[i] = '?'
+		}
+	}
+	return string(b)
+}
+
 // days words a duration in whole days, the unit these messages count in.
 func days(d time.Duration) string {
 	n := int(d.Hours() / 24)
@@ -49,7 +67,7 @@ func dnsProblem(hostname, domain string, faults []dnsFault, since time.Time) mes
 	lines := make([]string, len(faults))
 	for i, f := range faults {
 		names[i] = f.Record
-		lines[i] = f.Record + ": " + f.Detail
+		lines[i] = f.Record + ": " + plain(f.Detail, 600)
 	}
 	return compose(hostname,
 		fmt.Sprintf("%s: %s no longer matches", domain, strings.Join(names, ", ")),
@@ -91,7 +109,7 @@ func queueStuck(hostname string, old []postfix.QueuedMessage, now time.Time) mes
 		if sender == "" {
 			sender = "(bounce)"
 		}
-		fmt.Fprintf(&b, "%s  from %s  waiting %s, since %s\n", m.ID, sender, days(now.Sub(m.Arrival)), stamp(m.Arrival))
+		fmt.Fprintf(&b, "%s  from %s  waiting %s, since %s\n", plain(m.ID, 24), plain(sender, 120), days(now.Sub(m.Arrival)), stamp(m.Arrival))
 	}
 	what := "A message has"
 	if len(old) > 1 {