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/store/store.go

@@ -13,6 +13,7 @@ import (
 	"io/fs"
 	"os"
 	"sort"
+	"sync/atomic"
 
 	_ "modernc.org/sqlite" // pure-Go SQLite driver (no cgo), keeps the static build
 )
@@ -23,6 +24,10 @@ var migrationsFS embed.FS
 // Store wraps the database connection pool.
 type Store struct {
 	db *sql.DB
+
+	// inboundWrites counts journal rows written since the journal was last
+	// cut to its row cap, by both of its writers (countInboundWrites).
+	inboundWrites atomic.Int64
 }
 
 // Open opens (creating if needed) the SQLite database at path, enables WAL and