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/cluster/cluster_test.go

@@ -53,8 +53,12 @@ func (m *mockStore) GetNode(string) (models.ProbeNode, error) { return models.Pr
 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) SaveLog(string) error                     { return nil }
-func (m *mockStore) LoadLogs(int) ([]string, error)           { return nil, 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) SaveLog(string) error                           { return nil }
+func (m *mockStore) LoadLogs(int) ([]string, error)                 { return nil, nil }
 func (m *mockStore) GetActiveMaintenanceWindows() ([]models.MaintenanceWindow, error) {
 	return nil, nil
 }