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>

docs/architecture.md

@@ -149,7 +149,8 @@ smtp/inet` removes Debian's stock smtpd). Outbound delivery still uses the
   start are written only if the same row is not there already. The
   journal is pruned every six hours: rows
   older than the `inbound_spam_log_retention_days` setting, then down to
-  10 000.
+  10 000 - and down to 10 000 every 500 rows either writer adds in between
+  (the store counts them, `countInboundWrites`).
   **Filter lists** (`inbound_spam_list`): allow and deny entries for the
   external filter - sender address, sender domain, client address or network -
   validated and normalised by `internal/inbound` (`lists.go`) and written as