sender check: connections and messages counted apart; downtime estimated when supervisord is silent (stage 2b)

Following the design commit df96690.

- Two columns (owner, 2026-10-14): sender_check_days.messages (migration
  0009, not deployed) holds the messages deferred mid-message, from
  cleanup's warning; deferred stays the connections smtpd turned away.
  AddSenderCheckDeferred takes which; the counter passes cleanup's as a
  message. The card has "Connections turned away" and "Messages deferred";
  it warns on either within 24 hours (one mark, as before).
- Supervisord silent at start: postfixStarted asks again every 2 s for 30 s
  (postfixState: started, running, known). With no answer the downtime is
  the time since the alive mark - an upper bound - booked with estimated = 1
  (sender_check_days.estimated, MAX over a day); the card prefixes such a
  period's time with "≈ " and its note says what ≈ means. Before, such a
  start counted nothing and the number the owner decides by read low.
- Help topic sender-check and the log topic word for word as the mockup
  (help.go).

Tests: store (connections and messages apart, a period with an estimated
day); logtail (cleanup's warnings are messages); cmd/panel (supervisord
silent: the time since the mark, estimated); view fixture and the Health
page at four columns with ≈; e2e reads the row's two counts (panel down:
connections; panel hung: messages). Schema at 0009 with the columns.

schema-migrations row 9, CHANGELOG [Unreleased], audit plan lines ticked
(two), roadmap 28/62.
Evidence: go vet ./... clean; go test of store, view, cmd/panel ok locally,
logtail but for the Windows-only test; the server run follows before the
push.

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

test/e2e/sender_check_test.go

@@ -177,7 +177,8 @@ func testPanelDownDefersMail(t *testing.T, sc *scenario) {
 	}
 
 	// Server > Health, "Sender check", today: some downtime, one message.
-	today := regexp.MustCompile(`<tr><td>Today</td><td>([^<]*)</td><td>(\d+)</td></tr>`)
+	// Today: downtime, connections turned away, messages deferred mid-message.
+	today := regexp.MustCompile(`<tr><td>Today</td><td>([^<]*)</td><td>(\d+)</td><td>(\d+)</td></tr>`)
 	if err := waitFor("Health to show the downtime and the deferred message", 20*time.Second, 500*time.Millisecond, func() (bool, error) {
 		_, body, err := sc.panel.get("/server/health")
 		if err != nil {
@@ -201,17 +202,18 @@ func testPanelDownDefersMail(t *testing.T, sc *scenario) {
 }
 
 // senderCheckDeferredToday reads Server > Health's "Sender check" row for
-// today: the messages deferred because the check did not answer.
+// today: the messages deferred in the middle because the check stopped
+// answering (the last column; connections turned away are the one before).
 func senderCheckDeferredToday(sc *scenario) (int, error) {
 	_, body, err := sc.panel.get("/server/health")
 	if err != nil {
 		return 0, err
 	}
-	m := regexp.MustCompile(`<tr><td>Today</td><td>([^<]*)</td><td>(\d+)</td></tr>`).FindStringSubmatch(body)
+	m := regexp.MustCompile(`<tr><td>Today</td><td>([^<]*)</td><td>(\d+)</td><td>(\d+)</td></tr>`).FindStringSubmatch(body)
 	if m == nil {
 		return 0, fmt.Errorf("no Sender check row for today on Health")
 	}
-	return strconv.Atoi(m[2])
+	return strconv.Atoi(m[3])
 }
 
 // testPanelHungMidMessage: a panel that is running and stops answering in the