inbound-antispam-panel: refused inbound mail from mail.log; retention

Step 4/12 of docs/plans/inbound-antispam-panel.md.

The log tailer writes an inbound_spam_log row for mail port 25 refused
(internal/logtail/inbound.go): a "reject:" line of the port-25 smtpd is
Postfix's own policy; a milter-reject / milter-hold / milter-discard
line of cleanup is the filter's verdict. Only a recipient at an inbound
domain is a row, so "Relay access denied" for a stranger's domain is
not. The row is dated by its log line; the reply text is cut to 255
bytes of printable ASCII; a reply starting with 4 is a tempfail.

To tell port 25 from 465 / 587 the listener now logs as
postfix/inbound (syslog_name). cleanup logs under one name for every
listener, so its verdict counts only for a queue id the port-25 smtpd
was seen to open (4096 remembered). Lines are matched from the head,
after timestamp and host: their tail is the sender's own text.

Retention: PruneInboundSpam every six hours - older than the
inbound_spam_log_retention_days setting (seeded from
INBOUND_SPAM_LOG_RETENTION_DAYS, 90, range 7-365), then down to 10 000
rows. The setting has no field in the panel yet.

Known limits, in architecture.md: a log line names one recipient, so a
message refused at end-of-message is one row; a line re-read after an
unclean stop would be a second row.

Checked on the test server, uncommitted tree: go vet ./... ok,
go test ./... ok (22 packages), make e2e ok - the unknown recipient at
the inbound domain refused on port 25 has its row (reject by postfix),
the stranger's domain has none.

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

internal/store/settings.go

@@ -77,6 +77,72 @@ func (s *Store) EnsureSendLogRetentionDays(envDefault int) error {
 	return s.SetSendLogRetentionDays(sendLogRetentionFallback(envDefault))
 }
 
+const (
+	// InboundSpamLogRetentionDaysKey is the settings table key for the
+	// inbound antispam journal's retention. Same range as the send log's.
+	InboundSpamLogRetentionDaysKey = "inbound_spam_log_retention_days"
+	InboundSpamLogRetentionDaysMin = 7
+	InboundSpamLogRetentionDaysMax = 365
+	// InboundSpamLogRetentionDaysDefault matches
+	// INBOUND_SPAM_LOG_RETENTION_DAYS when unset.
+	InboundSpamLogRetentionDaysDefault = 90
+)
+
+// ErrInboundSpamLogRetentionDaysOutOfRange is returned when the inbound
+// journal's retention is outside the allowed panel range.
+var ErrInboundSpamLogRetentionDaysOutOfRange = errors.New("inbound journal retention days out of range")
+
+// ValidateInboundSpamLogRetentionDays checks the panel-allowed window.
+func ValidateInboundSpamLogRetentionDays(days int) error {
+	if days < InboundSpamLogRetentionDaysMin || days > InboundSpamLogRetentionDaysMax {
+		return fmt.Errorf("%w: must be between %d and %d days", ErrInboundSpamLogRetentionDaysOutOfRange, InboundSpamLogRetentionDaysMin, InboundSpamLogRetentionDaysMax)
+	}
+	return nil
+}
+
+func inboundSpamLogRetentionFallback(envDefault int) int {
+	if ValidateInboundSpamLogRetentionDays(envDefault) == nil {
+		return envDefault
+	}
+	return InboundSpamLogRetentionDaysDefault
+}
+
+// GetInboundSpamLogRetentionDays returns the effective retention window of
+// the inbound journal. When the setting is missing or invalid, envDefault is
+// used (INBOUND_SPAM_LOG_RETENTION_DAYS).
+func (s *Store) GetInboundSpamLogRetentionDays(envDefault int) (int, error) {
+	raw, err := s.GetSetting(InboundSpamLogRetentionDaysKey)
+	if err != nil {
+		return 0, err
+	}
+	days, err := strconv.Atoi(raw)
+	if err != nil || ValidateInboundSpamLogRetentionDays(days) != nil {
+		return inboundSpamLogRetentionFallback(envDefault), nil
+	}
+	return days, nil
+}
+
+// SetInboundSpamLogRetentionDays persists the inbound journal's retention.
+func (s *Store) SetInboundSpamLogRetentionDays(days int) error {
+	if err := ValidateInboundSpamLogRetentionDays(days); err != nil {
+		return err
+	}
+	return s.SetSetting(InboundSpamLogRetentionDaysKey, strconv.Itoa(days))
+}
+
+// EnsureInboundSpamLogRetentionDays seeds the setting from envDefault when it
+// has never been written.
+func (s *Store) EnsureInboundSpamLogRetentionDays(envDefault int) error {
+	raw, err := s.GetSetting(InboundSpamLogRetentionDaysKey)
+	if err != nil {
+		return err
+	}
+	if raw != "" {
+		return nil
+	}
+	return s.SetInboundSpamLogRetentionDays(inboundSpamLogRetentionFallback(envDefault))
+}
+
 // GetSetting returns a settings value or empty string when missing.
 func (s *Store) GetSetting(key string) (string, error) {
 	var value string