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/setup.sh

@@ -9,13 +9,16 @@ echo "==> Seeding monitors and alerts..."
 UPTOP_DB_DSN="$DB" ./uptop apply -f vhs/seed.yaml 2>&1
 
 echo "==> Backfilling check history..."
-BACKFILL_OUT=$(go run ./vhs/backfill/ "$DB")
+# Build first so the backfill's `now` (node last_seen, heartbeat timing) isn't racing
+# a cold compile — keeps the capture window deterministic.
+go build -o /tmp/uptop-backfill ./vhs/backfill/
+BACKFILL_OUT=$(/tmp/uptop-backfill "$DB")
 echo "$BACKFILL_OUT"
 
 PUSH_TOKEN=$(echo "$BACKFILL_OUT" | grep '^PUSH_TOKEN=' | cut -d= -f2)
 if [ -n "$PUSH_TOKEN" ]; then
-  echo "==> Sending push heartbeat in 15s (background)..."
-  (sleep 15 && curl -s "http://localhost:18099/api/push" -H "Authorization: Bearer $PUSH_TOKEN" > /dev/null 2>&1) &
+  echo "==> Sending push heartbeat in 10s (background)..."
+  (sleep 10 && curl -s "http://localhost:18099/api/push" -H "Authorization: Bearer $PUSH_TOKEN" > /dev/null 2>&1) &
 fi
 
 echo "==> Starting uptop server..."
@@ -24,4 +27,5 @@ exec env \
   UPTOP_PORT=23299 \
   UPTOP_HTTP_PORT=18099 \
   UPTOP_ALLOW_PRIVATE_TARGETS=true \
+  UPTOP_DEMO=1 \
   ./uptop serve 2>/dev/null