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>

deploy/docker-compose.yml

@@ -47,6 +47,7 @@ services:
       # How long the send log keeps rows before the background sweep deletes
       # them — the main driver of /data growth over time.
       SEND_LOG_RETENTION_DAYS: "${SEND_LOG_RETENTION_DAYS:-90}"
+      INBOUND_SPAM_LOG_RETENTION_DAYS: "${INBOUND_SPAM_LOG_RETENTION_DAYS:-90}"
       # Sliding idle timeout for the panel login session, in days.
       PANEL_SESSION_IDLE_DAYS: "${PANEL_SESSION_IDLE_DAYS:-7}"
       # Resolvers the panel's PTR/SPF/DKIM/DMARC checks query directly, so they