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/seed.yaml

@@ -28,7 +28,7 @@ monitors:
   # HTTP — homelab services
   - name: Nextcloud
     type: http
-    url: https://example.com
+    url: https://nextcloud.com
     interval: 30
     alert: Discord Homelab
     check_ssl: true
@@ -37,21 +37,21 @@ monitors:
 
   - name: Jellyfin
     type: http
-    url: https://example.com
+    url: https://jellyfin.org
     interval: 30
     alert: Discord Homelab
     max_retries: 2
 
   - name: Home Assistant
     type: http
-    url: https://example.com
+    url: https://www.home-assistant.io
     interval: 30
     alert: Discord Homelab
     max_retries: 3
 
   - name: Gitea
     type: http
-    url: https://example.com
+    url: https://about.gitea.com
     interval: 60
     alert: Discord Homelab
     check_ssl: true
@@ -60,14 +60,14 @@ monitors:
 
   - name: Traefik Dashboard
     type: http
-    url: https://example.com
+    url: https://traefik.io
     interval: 60
     alert: Discord Homelab
     max_retries: 1
 
   - name: Vaultwarden
     type: http
-    url: https://example.com
+    url: https://bitwarden.com
     interval: 30
     alert: Discord Homelab
     check_ssl: true
@@ -76,7 +76,7 @@ monitors:
 
   - name: Personal Blog
     type: http
-    url: https://example.com
+    url: https://jvns.ca
     interval: 120
     alert: Discord Homelab
     check_ssl: true
@@ -85,17 +85,17 @@ monitors:
 
   - name: Immich
     type: http
-    url: https://example.com
+    url: https://immich.app
     interval: 60
     alert: Discord Homelab
     check_ssl: true
     expiry_threshold: 7
     max_retries: 3
 
-  # HTTP — deliberate failure
+  # HTTP — deliberate failure (non-resolving homelab host → stays DOWN)
   - name: Auth Portal
     type: http
-    url: http://localhost:1
+    url: https://auth.home.arpa
     interval: 30
     alert: Discord Homelab
     max_retries: 2