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/tab_sites.go

@@ -959,19 +959,26 @@ func (m Model) viewDetailPanel() string {
 		}
 	} else {
 		b.WriteString("  " + latencySparkline(hist.Latencies, hist.Statuses, sparkWidth))
-		if len(hist.Latencies) > 0 {
-			minL, maxL := hist.Latencies[0], hist.Latencies[0]
-			var total time.Duration
-			for _, l := range hist.Latencies {
-				total += l
-				if l < minL {
-					minL = l
-				}
-				if l > maxL {
-					maxL = l
-				}
+		// Stats over successful checks only — a failed check is stored as 0ns latency
+		// and would otherwise drag Min to 0ms and skew the average.
+		var minL, maxL, total time.Duration
+		count := 0
+		for i, l := range hist.Latencies {
+			if i < len(hist.Statuses) && !hist.Statuses[i] {
+				continue
 			}
-			avg := total / time.Duration(len(hist.Latencies))
+			if count == 0 {
+				minL, maxL = l, l
+			} else if l < minL {
+				minL = l
+			} else if l > maxL {
+				maxL = l
+			}
+			total += l
+			count++
+		}
+		if count > 0 {
+			avg := total / time.Duration(count)
 			fmt.Fprintf(&b, "\n  %s %dms  %s %dms  %s %dms",
 				subtleStyle.Render("Min"), minL.Milliseconds(),
 				subtleStyle.Render("Avg"), avg.Milliseconds(),