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/fixtures_test.go

@@ -96,9 +96,9 @@ func healthFixture(refresh bool) *Health {
 		h.AsFragment()
 	}
 	h.WithSenderCheck([]SenderCheckRow{
-		{Period: "Today", Down: "0 s", Deferred: "0"},
-		{Period: "Last 30 days", Down: "4 s", Deferred: "0"},
-		{Period: "Last 12 months", Down: "41 s", Deferred: "3"},
+		{Period: "Today", Down: "0 s", Deferred: "0", Messages: "0"},
+		{Period: "Last 30 days", Down: "4 s", Deferred: "0", Messages: "0"},
+		{Period: "Last 12 months", Down: "≈ 41 s", Deferred: "3", Messages: "1"},
 	}, "2026-10-13", "ok")
 	h.Machine.End = Verdict("ok")
 	h.MachineRows = []MachineRow{