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

vhs/demo.tape

@@ -6,49 +6,67 @@ Set Padding 20
 Set Framerate 15
 Set TypingSpeed 50ms
 
+# Seed demo data + start uptop (UPTOP_DEMO=1 → stable pulse dot for stills).
 Hide
 Type "bash vhs/setup.sh /tmp/uptop-vhs.db"
 Enter
-Sleep 45s
+# Warm-up: push heartbeat lands (~10s) and initial checks settle. Kept short so every
+# capture stays inside the 60s node-freshness window (consistent "3 probes" footer).
+Sleep 18s
 Show
-Sleep 5s
+Sleep 2s
 
-# Sites tab — hero shot with mixed monitor states
+# 1. Sites — hero shot: mixed states, history sparklines, SSL, retries.
 Screenshot vhs/screenshots/monitors.png
 Sleep 1s
 
-# Navigate to Nextcloud (row 6: group + 3 children + Auth Portal)
+# 2. Detail — drill into Nextcloud (6th row from the top).
 Down
-Sleep 200ms
+Sleep 150ms
 Down
-Sleep 200ms
+Sleep 150ms
 Down
-Sleep 200ms
+Sleep 150ms
 Down
-Sleep 200ms
+Sleep 150ms
 Down
-Sleep 200ms
+Sleep 300ms
 Type "i"
-Sleep 3s
+Sleep 2s
 Screenshot vhs/screenshots/detail.png
-Sleep 1s
-
-# Close detail
+Sleep 500ms
 Escape
 Sleep 1s
 
-# Tab to Alerts
+# 3. Alerts — channels with health dots + recent "last sent".
 Tab
-Sleep 2s
+Sleep 1500ms
 Screenshot vhs/screenshots/alerts.png
-Sleep 1s
+Sleep 500ms
 
-# Tab to Logs
+# 4. Logs — chronological, severity-coloured event stream.
 Tab
-Sleep 2s
+Sleep 1500ms
 Screenshot vhs/screenshots/logs.png
-Sleep 1s
+Sleep 500ms
+
+# 5. Nodes — distributed probes across regions.
+Tab
+Sleep 1500ms
+Screenshot vhs/screenshots/nodes.png
+Sleep 500ms
+
+# 6. Theme — cycle to the next theme, return to Sites for an alternate-palette hero.
+Type "T"
+Sleep 500ms
+Tab
+Sleep 200ms
+Tab
+Sleep 200ms
+Tab
+Sleep 1s
+Screenshot vhs/screenshots/theme.png
+Sleep 500ms
 
-# Quit
 Type "q"
 Sleep 1s