chore: add TUI screenshots via VHS with realistic seed data

Screenshots capture 4 views: monitors dashboard (hero), detail panel,
alerts tab, and logs tab. Seed data uses homelab-themed monitors with
a SQL backfill for rich sparkline history, state changes, and log
entries.

Also fixes latencySparkline to color DOWN checks red instead of green
— previously failed checks with 0ms latency rendered as green bars.

vhs/setup.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+# VHS screenshot setup: seed monitors, backfill history, start server.
+set -e
+DB="${1:?usage: setup.sh <db-path>}"
+
+rm -f "$DB" "$DB-shm" "$DB-wal"
+
+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/ "$DB")
+echo "$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 $PUSH_TOKEN" > /dev/null 2>&1) &
+fi
+
+echo "==> Starting uptop server..."
+exec env \
+  UPTOP_DB_DSN="$DB" \
+  UPTOP_PORT=23299 \
+  UPTOP_HTTP_PORT=18099 \
+  UPTOP_ALLOW_PRIVATE_TARGETS=true \
+  ./uptop serve 2>/dev/null