inbound journal: one row per recipient, and every row counts to the cap (audit M3)

Security audit 2026-10-17, finding M3, stage 1. inboundSession.RcptTo
appended every RCPT (the outbound session de-duplicates), and the end of
the message inserted a row per recipient one statement at a time inside
Postfix's callback; the cut to 10 000 rows every 500 counted only the log
tailer's rows. Postfix accepts a repeated recipient up to
smtpd_recipient_limit (1 000) and sends each to the milter, so one message
was a thousand rows, uncut until the six-hourly prune.

Now: the inbound milter folds a repeated recipient (case-insensitive, as
cleanup's been_here() with BH_FLAG_FOLD does, and as session.RcptTo does);
a message's rows go in one transaction (store.InsertInboundSpamMessage,
which replaces InsertInboundSpam in milter.InboundStore); the count and the
cut moved from the tailer into the store (countInboundWrites, an atomic
counter on Store) so the milter's rows, the tailer's and the tailer's
start-up re-reads all count. store.InboundSpamLogMaxRows is the one cap.

Tests: TestInboundRecipientRepeatedIsOneRow (1 000 RCPT in two cases, one
row); TestInboundJournalIsCutWhicheverWriterWrites (a small cap: a message's
rows and refusals together cross the mark, the table is cut); the tailer's
own counting test went with its counter. go test ./internal/store
./internal/milter ./internal/logtail (Windows-only Follow failures aside).
Also gofmt of a test line from the M2 commit.

Docs: security.md § Mail path, architecture.md; CHANGELOG; plan.

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

internal/milter/inbound_test.go

@@ -20,11 +20,14 @@ type fakeInbound struct {
 	forgot    []string
 }
 
-func (f *fakeInbound) InsertInboundSpam(e store.InboundSpamEntry) error {
+func (f *fakeInbound) InsertInboundSpamMessage(e store.InboundSpamEntry, rcpts []store.InboundRecipient) error {
 	if f.insertErr != nil {
 		return f.insertErr
 	}
-	f.rows = append(f.rows, e)
+	for _, r := range rcpts {
+		e.InboundDomain, e.To = r.Domain, r.To
+		f.rows = append(f.rows, e)
+	}
 	return nil
 }
 
@@ -400,3 +403,23 @@ func TestInboundRowsMeetTheirDeliveryLines(t *testing.T) {
 		t.Errorf("with no queue id: rows %+v, detached %q", rec.rows, rec.forgot)
 	}
 }
+
+// A recipient given again - in any case - is one row: Postfix accepts a
+// repeated RCPT up to smtpd_recipient_limit (1 000) and sends each, and one
+// message used to be a thousand rows (security audit 2026-10-17, M3).
+func TestInboundRecipientRepeatedIsOneRow(t *testing.T) {
+	rec := &fakeInbound{domains: map[string]bool{"in.example.com": true}}
+	s := inboundSessionFor(t, rec, false)
+	rcpts := make([]string, 0, 1000)
+	for i := 0; i < 1000; i++ {
+		if i%2 == 0 {
+			rcpts = append(rcpts, "listed@in.example.com")
+		} else {
+			rcpts = append(rcpts, "LISTED@in.example.com")
+		}
+	}
+	message(t, s, "sender@example.net", rcpts, nil)
+	if len(rec.rows) != 1 || rec.rows[0].To != "listed@in.example.com" {
+		t.Errorf("rows = %d (%+v), want one, spelt as first given", len(rec.rows), rec.rows)
+	}
+}