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>

internal/web/handlers/handlers_status.go

@@ -152,7 +152,7 @@ func (h *Handlers) healthView(r *http.Request, flash string) *view.Health {
 	if err != nil {
 		// Zeros that were not read are not zeros.
 		for i := range rows {
-			rows[i].Down, rows[i].Deferred = "—", "—"
+			rows[i].Down, rows[i].Deferred, rows[i].Messages = "—", "—", "—"
 		}
 		status = string(health.StatusUnknown)
 	}
@@ -160,11 +160,17 @@ func (h *Handlers) healthView(r *http.Request, flash string) *view.Health {
 }
 
 // senderCheckCard is the Sender check card from its statistics. It warns when
-// a message was deferred in the last 24 hours: that is mail an application had
-// to send again. Downtime that deferred nothing is only shown.
+// a connection was turned away or a message deferred in the last 24 hours:
+// that is mail an application had to send again. Downtime that deferred
+// nothing is only shown.
 func senderCheckCard(s store.SenderCheckStats, now time.Time) (rows []view.SenderCheckRow, since, status string) {
 	row := func(period string, t store.SenderCheckTotals) view.SenderCheckRow {
-		return view.SenderCheckRow{Period: period, Down: view.Downtime(t.Down), Deferred: strconv.FormatInt(t.Deferred, 10)}
+		down := view.Downtime(t.Down)
+		if t.Estimated {
+			down = "≈ " + down
+		}
+		return view.SenderCheckRow{Period: period, Down: down,
+			Deferred: strconv.FormatInt(t.Deferred, 10), Messages: strconv.FormatInt(t.Messages, 10)}
 	}
 	rows = []view.SenderCheckRow{row("Today", s.Today), row("Last 30 days", s.Month), row("Last 12 months", s.Year)}
 	if !s.Since.IsZero() {