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/store/schema_test.go

@@ -59,7 +59,7 @@ func TestBaselineSchema(t *testing.T) {
 		"inbound_spam_list":     "id list_type entry_type value note created_at",
 		"send_log_lines":        "id queue_id logged_at line",
 		"notify_state":          "key phase count mark since sent_at",
-		"sender_check_days":     "day down_seconds deferred",
+		"sender_check_days":     "day down_seconds deferred messages estimated",
 		"delivery_log_days":     "day reconcile_closed reconcile_missed reconcile_contradicted tailer_blind_seconds queued_over_lifetime",
 	}
 	rows, err := st.db.Query("SELECT name FROM sqlite_master WHERE type = 'table' AND name NOT LIKE 'sqlite_%' ORDER BY name")