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/view/pages_test.go

@@ -177,7 +177,7 @@ func TestHealthShowsEveryCheckAndBothActions(t *testing.T) {
 			t.Errorf("Health is missing %s", want)
 		}
 	}
-	pageHas(t, "Health", out, `<tr><td>Last 12 months</td><td>41 s</td><td>3</td></tr>`,
+	pageHas(t, "Health", out, `<tr><td>Last 12 months</td><td>≈ 41 s</td><td>3</td><td>1</td></tr>`,
 		`Counted since <span class="sp-mono">2026-10-13</span>.`, `href="/help#sender-check"`)
 	// The Configuration box is outside the polled stack: a refresh must not re-render a control.
 	if strings.Contains(out[strings.Index(out, `id="health-stack"`):strings.Index(out, `id="sockets"`)], `action="/server/health/reload"`) {