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>

build/postfix-config.sh

@@ -412,8 +412,12 @@ if [ "${INBOUND_RELAY_ENABLE}" = "true" ] || [ "${DMARC_REPORTS_ENABLE}" = "true
 		fi
 	fi
 
+	# syslog_name: this listener's lines read postfix/inbound/smtpd, the
+	# submission ports' postfix/smtpd. The log tailer files a refusal on port 25
+	# into the inbound journal and has to tell the two apart.
 	postconf -M "smtp/inet=smtp inet n - n - - smtpd"
 	postconf -P \
+		"smtp/inet/syslog_name=postfix/inbound" \
 		"smtp/inet/smtpd_sasl_auth_enable=no" \
 		"smtp/inet/smtpd_tls_auth_only=no" \
 		"smtp/inet/smtpd_sender_login_maps=" \