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/tui/tui.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"math"
+	"os"
 	"sort"
 	"strings"
 	"time"
@@ -122,6 +123,10 @@ type Model struct {
 
 	filterMode bool
 	filterText string
+
+	// demoMode renders a stable status dot instead of the animated pulse so
+	// screenshots/recordings don't capture the spinner mid-frame. Set via UPTOP_DEMO=1.
+	demoMode bool
 }
 
 func InitialModel(isAdmin bool, s store.Store, eng *monitor.Engine) Model {
@@ -155,6 +160,7 @@ func InitialModel(isAdmin bool, s store.Store, eng *monitor.Engine) Model {
 		collapsed:    collapsed,
 		theme:        theme,
 		themeIndex:   themeIdx,
+		demoMode:     os.Getenv("UPTOP_DEMO") == "1",
 	}
 }
 
@@ -754,11 +760,6 @@ func (m *Model) submitForm() {
 }
 
 func (m Model) pulseIndicator() string {
-	frame := m.tickCount % len(pulseFrames)
-	brightness := int(m.pulsePos*155) + 100
-	if brightness > 255 {
-		brightness = 255
-	}
 	hasDown := false
 	for _, s := range m.sites {
 		if !s.Paused && !m.isMonitorInMaintenance(s.ID) && (s.Status == "DOWN" || s.Status == "SSL EXP") {
@@ -766,6 +767,19 @@ func (m Model) pulseIndicator() string {
 			break
 		}
 	}
+	// Stills can't show animation: render a stable status dot in demo mode.
+	if m.demoMode {
+		c := m.theme.Success
+		if hasDown {
+			c = m.theme.Danger
+		}
+		return lipgloss.NewStyle().Foreground(c).Render("●")
+	}
+	frame := m.tickCount % len(pulseFrames)
+	brightness := int(m.pulsePos*155) + 100
+	if brightness > 255 {
+		brightness = 255
+	}
 	var color string
 	if hasDown {
 		color = fmt.Sprintf("#%02x%02x%02x", brightness, brightness/4, brightness/4)
@@ -953,7 +967,11 @@ func (m Model) viewDashboard() string {
 				online++
 			}
 		}
-		statusParts = append(statusParts, fmt.Sprintf("%d probes", online))
+		probeLabel := "probes"
+		if online == 1 {
+			probeLabel = "probe"
+		}
+		statusParts = append(statusParts, fmt.Sprintf("%d %s", online, probeLabel))
 	}
 	statusLine := strings.Join(statusParts, subtleStyle.Render(" · "))