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>

docs/architecture.md

@@ -89,6 +89,19 @@ smtp/inet` removes Debian's stock smtpd). Outbound delivery still uses the
   address is not one - and never refuses anything
   ([internal/milter/inbound.go](../internal/milter/inbound.go)). The headers
   are read only while a filter is attached; refused mail never reaches it.
+  Refusals come from `mail.log`: this listener logs as `postfix/inbound`
+  (`syslog_name`), and the log tailer turns a `reject:` line of
+  `postfix/inbound/smtpd` (Postfix's own policy) and a `milter-reject` /
+  `milter-hold` / `milter-discard` line of `cleanup` (the filter's verdict,
+  counted only for a queue id that listener opened) into a row, dated by the
+  line, when the recipient is at an inbound domain
+  ([internal/logtail/inbound.go](../internal/logtail/inbound.go)). A line
+  names one recipient, so a message refused at end-of-message is one row
+  whatever its recipient count, and a line read twice (the tailer saves its
+  place once a second; an unclean stop can lose that second) is two rows. The
+  journal is pruned every six hours: rows
+  older than the `inbound_spam_log_retention_days` setting, then down to
+  10 000.
   **Inbound relay** (`INBOUND_RELAY_ENABLE=true`): accepts only
   `relay_domains` + `relay_recipient_maps` (`reject_unauth_destination`,
   `reject_unlisted_recipient`). Maps under `/data/postfix/` (`relay_domains`,