chore(tui): polish demo + regenerate screenshots

Rework the VHS demo so the README screenshots actually entice a download.

Demo data / tooling:
- seed.yaml: real, reachable service URLs (detail now shows nextcloud.com,
  not example.com); Auth Portal -> non-resolving home.arpa host so it reads
  as a believable, reliably-DOWN monitor
- backfill: transient outages for Nextcloud/Jellyfin/Immich aligned with their
  state changes (uptime % now matches); log timestamps derived from now so the
  Logs view reads chronologically; real SSL warning; three probe nodes across
  regions; seeded alert send health
- demo.tape: shorter warm-up, added Nodes + theme captures, ordered so every
  shot stays inside the 60s node-freshness window (consistent probe count)
- vhs/crop: new tool to trim the empty terminal border around each screenshot
- setup.sh: build backfill up front for deterministic timing; UPTOP_DEMO=1

Supporting code:
- persist alert send health (new alert_health table, load on startup,
  best-effort save on send) so health/last-sent survive restarts
- latency Min/Avg/Max ignore failed checks (no more "Min 0ms")
- correct "probe"/"probes" pluralization
- stable status dot instead of an animated spinner under UPTOP_DEMO

internal/monitor/monitor.go

@@ -146,6 +146,26 @@ func (e *Engine) InitLogs() {
 	e.logStore = logs
 }
 
+// InitAlertHealth restores persisted alert send health so the dashboard shows real
+// "last sent" / health state on startup instead of resetting every channel to "never".
+func (e *Engine) InitAlertHealth() {
+	records, err := e.db.LoadAlertHealth()
+	if err != nil {
+		return
+	}
+	e.alertHealthMu.Lock()
+	defer e.alertHealthMu.Unlock()
+	for id, r := range records {
+		e.alertHealth[id] = AlertHealth{
+			LastSendAt: r.LastSendAt,
+			LastSendOK: r.LastSendOK,
+			LastError:  r.LastError,
+			SendCount:  r.SendCount,
+			FailCount:  r.FailCount,
+		}
+	}
+}
+
 func (e *Engine) GetLogs() []string {
 	e.logMu.RLock()
 	defer e.logMu.RUnlock()
@@ -612,6 +632,18 @@ func (e *Engine) recordAlertResult(alertID int, ok bool, errMsg string) {
 		h.FailCount++
 	}
 	e.alertHealth[alertID] = h
+
+	// Persist best-effort so health survives restarts; DB IO off the alert path.
+	go func(rec models.AlertHealthRecord) {
+		_ = e.db.SaveAlertHealth(rec)
+	}(models.AlertHealthRecord{
+		AlertID:    alertID,
+		LastSendAt: h.LastSendAt,
+		LastSendOK: h.LastSendOK,
+		LastError:  h.LastError,
+		SendCount:  h.SendCount,
+		FailCount:  h.FailCount,
+	})
 }
 
 func (e *Engine) GetAlertHealth(alertID int) AlertHealth {

internal/monitor/monitor_test.go

@@ -63,6 +63,10 @@ func (m *mockStore) GetNode(string) (models.ProbeNode, error)         { return m
 func (m *mockStore) GetAllNodes() ([]models.ProbeNode, error)         { return nil, nil }
 func (m *mockStore) UpdateNodeLastSeen(string) error                  { return nil }
 func (m *mockStore) DeleteNode(string) error                          { return nil }
+func (m *mockStore) LoadAlertHealth() (map[int]models.AlertHealthRecord, error) {
+	return nil, nil
+}
+func (m *mockStore) SaveAlertHealth(models.AlertHealthRecord) error { return nil }
 func (m *mockStore) GetActiveMaintenanceWindows() ([]models.MaintenanceWindow, error) {
 	return nil, nil
 }