Add server-side health checks for service online status

- update-health.js: pings every service URL from script.js (single
  source of truth) in parallel every minute via cron and writes
  health.json; any HTTP response below 500 counts as online, 5xx as
  error, timeouts/DNS failures as offline; TLS validity ignored
- Card badges now show real state instead of a static Online label:
  grey Checking, green Online (with latency tooltip), amber Error
  (HTTP code), red Offline (reason); refreshed every 30 s
- Entrypoint runs a first sweep at boot; nginx serves /health.json
  with no-cache; README documents the behavior

docker-entrypoint.sh

@@ -13,5 +13,8 @@ node /opt/dashboard/server.js &
 # Run the initial data fetch in the background; cron keeps it fresh after that.
 ( cd /opt/dashboard && node update-data.js ) &
 
+# First health-check sweep in the background; cron re-runs it every minute.
+( cd /opt/dashboard && node update-health.js ) &
+
 # Start nginx in the foreground.
 exec nginx -g "daemon off;"