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/inboundspam_test.go

@@ -59,3 +59,102 @@ func TestIsInboundDomain(t *testing.T) {
 		}
 	}
 }
+
+func TestInsertInboundSpamKeepsItsOwnTime(t *testing.T) {
+	st := openTestStore(t)
+	at := time.Date(2026, 10, 1, 9, 30, 0, 0, time.FixedZone("msk", 3*3600))
+	err := st.InsertInboundSpam(InboundSpamEntry{
+		InboundDomain: "in.example.com", ClientIP: "203.0.113.7", To: "a@in.example.com",
+		Decision: SpamDecisionReject, Engine: SpamEnginePostfix, At: at,
+	})
+	if err != nil {
+		t.Fatalf("InsertInboundSpam: %v", err)
+	}
+	var created string
+	if err := st.db.QueryRow("SELECT created_at FROM inbound_spam_log").Scan(&created); err != nil || created != "2026-10-01T06:30:00Z" {
+		t.Errorf("created_at = %q, %v", created, err)
+	}
+}
+
+func TestPruneInboundSpam(t *testing.T) {
+	st := openTestStore(t)
+	now := time.Now().UTC()
+	add := func(to string, age time.Duration) {
+		t.Helper()
+		err := st.InsertInboundSpam(InboundSpamEntry{
+			InboundDomain: "in.example.com", ClientIP: "203.0.113.7", To: to,
+			Decision: SpamDecisionReject, Engine: SpamEnginePostfix, At: now.Add(-age),
+		})
+		if err != nil {
+			t.Fatalf("InsertInboundSpam: %v", err)
+		}
+	}
+	left := func() string {
+		t.Helper()
+		rows, err := st.db.Query("SELECT to_addr FROM inbound_spam_log ORDER BY id")
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer rows.Close()
+		var out string
+		for rows.Next() {
+			var to string
+			if err := rows.Scan(&to); err != nil {
+				t.Fatal(err)
+			}
+			out += to + " "
+		}
+		return out
+	}
+
+	add("old", 100*24*time.Hour)
+	add("a", 3*time.Hour)
+	add("b", 2*time.Hour)
+	add("c", time.Hour)
+
+	// Age first: nothing else goes while the rest fits.
+	if n, err := st.PruneInboundSpam(now.AddDate(0, 0, -90), 10); err != nil || n != 1 {
+		t.Fatalf("prune by age = %d, %v", n, err)
+	}
+	if got := left(); got != "a b c " {
+		t.Fatalf("left = %q", got)
+	}
+	// Then the cap, oldest first.
+	if n, err := st.PruneInboundSpam(now.AddDate(0, 0, -90), 2); err != nil || n != 1 {
+		t.Fatalf("prune by count = %d, %v", n, err)
+	}
+	if got := left(); got != "b c " {
+		t.Fatalf("left = %q", got)
+	}
+	if n, err := st.PruneInboundSpam(now.AddDate(0, 0, -90), 2); err != nil || n != 0 {
+		t.Errorf("a second prune removed %d, %v", n, err)
+	}
+}
+
+func TestInboundSpamLogRetentionDays(t *testing.T) {
+	st := openTestStore(t)
+	if days, err := st.GetInboundSpamLogRetentionDays(30); err != nil || days != 30 {
+		t.Errorf("unset, env 30: %d, %v", days, err)
+	}
+	if days, err := st.GetInboundSpamLogRetentionDays(3); err != nil || days != InboundSpamLogRetentionDaysDefault {
+		t.Errorf("unset, env out of range: %d, %v", days, err)
+	}
+	if err := st.EnsureInboundSpamLogRetentionDays(30); err != nil {
+		t.Fatalf("Ensure: %v", err)
+	}
+	if err := st.EnsureInboundSpamLogRetentionDays(60); err != nil {
+		t.Fatalf("Ensure again: %v", err)
+	}
+	if days, _ := st.GetInboundSpamLogRetentionDays(90); days != 30 {
+		t.Errorf("after Ensure(30), Ensure(60): %d, want the first seed kept", days)
+	}
+	if err := st.SetInboundSpamLogRetentionDays(6); err == nil {
+		t.Error("6 days was accepted")
+	}
+	if err := st.SetInboundSpamLogRetentionDays(365); err != nil {
+		t.Errorf("365 days: %v", err)
+	}
+	if days, _ := st.GetInboundSpamLogRetentionDays(90); days != 365 {
+		t.Errorf("after Set(365): %d", days)
+	}
+}