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

README.md

@@ -21,7 +21,9 @@ External: gitlab-ixsol, gem360.
   with their GPU/CPU split)
 - Inline **Open WebUI prompt line with model selection** in the Open WebUI card
   (requires `OPENWEBUI_TOKEN`)
-- Responsive grid of service cards with icons, categories and a live "Online" indicator
+- Responsive grid of service cards with icons, categories and **real online status**
+  (server-side health checks every minute: green = online, amber = 5xx error,
+  red = unreachable; hover the badge for latency/details)
 - **Open** (new tab) + **Copy URL** per service
 - Live search / filter across names, categories and URLs
 - Live local clock
@@ -101,9 +103,12 @@ docker run -d --name homelab-dashboard -p 8888:80 --restart unless-stopped homel
 
 ## Notes
 
-- The "Online" badge is cosmetic (a static site can't probe your services from the browser
-  due to CORS). If you want real health checks later, a tiny backend that pings each URL
-  would be the next step.
+- Online badges are real health checks: `api/update-health.js` (cron, every minute)
+  pings each service URL from the container with a 5 s timeout and writes
+  `health.json`; the frontend refreshes badges every 30 s. Any HTTP response below
+  500 counts as online (401/404 still means the server is up); 5xx shows as
+  "Error", timeouts/DNS failures as "Offline". TLS certificate validity is ignored
+  (reachability only).
 - Calendar dependencies are bundled in `api/node_modules` so the Docker build works
   offline. To update them, run `npm install` in the `api/` directory.
 - Port 8888 is published by the Compose file; change it there if needed.