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

api/update-health.js

@@ -0,0 +1,111 @@
+// ---------------------------------------------------------------------------
+// Health checks for all dashboard services.
+// Reads the service URLs from script.js (single source of truth), pings each
+// one in parallel and writes health.json. Scheduled every minute by cron.
+//
+// Status classification:
+//   online  - got any HTTP response below 500 (401/404 still means "up")
+//   error   - server responded with 5xx
+//   offline - timeout / connection refused / DNS failure
+// TLS certificate validity is ignored (reachability check only).
+// ---------------------------------------------------------------------------
+
+const http = require("http");
+const https = require("https");
+const fs = require("fs");
+const path = require("path");
+const { URL } = require("url");
+
+const OUT_FILE = process.env.OUT_FILE || "/usr/share/nginx/html/health.json";
+const TIMEOUT = parseInt(process.env.HEALTH_TIMEOUT || "5000", 10);
+
+// Reachability only — don't fail on self-signed certificates.
+const httpAgent = new http.Agent({ rejectUnauthorized: false });
+const httpsAgent = new https.Agent({ rejectUnauthorized: false });
+
+function serviceUrls() {
+  const candidates = [
+    process.env.SERVICES_JS,
+    "/usr/share/nginx/html/script.js",
+    path.join(__dirname, "..", "script.js"),
+  ].filter(Boolean);
+  for (const p of candidates) {
+    try {
+      const src = fs.readFileSync(p, "utf8");
+      const urls = [...src.matchAll(/url: *"(https?:\/\/[^"]+)"/g)].map((m) => m[1]);
+      if (urls.length) return [...new Set(urls)];
+    } catch {
+      // try next candidate
+    }
+  }
+  return [];
+}
+
+function checkUrl(url) {
+  return new Promise((resolve) => {
+    const started = Date.now();
+    let settled = false;
+    const done = (result) => {
+      if (settled) return;
+      settled = true;
+      resolve(result);
+    };
+
+    let req;
+    try {
+      const u = new URL(url);
+      const client = u.protocol === "https:" ? https : http;
+      req = client.request(
+        u,
+        {
+          method: "GET",
+          agent: u.protocol === "https:" ? httpsAgent : httpAgent,
+          timeout: TIMEOUT,
+        },
+        (res) => {
+          const code = res.statusCode;
+          res.destroy(); // response headers are all we need
+          done({
+            status: code >= 500 ? "error" : "online",
+            code,
+            latencyMs: Date.now() - started,
+            error: null,
+          });
+        }
+      );
+    } catch (e) {
+      return done({ status: "offline", code: null, latencyMs: null, error: e.message });
+    }
+
+    req.on("timeout", () => {
+      req.destroy();
+      done({ status: "offline", code: null, latencyMs: null, error: "Timeout" });
+    });
+    req.on("error", (e) => {
+      done({ status: "offline", code: null, latencyMs: null, error: e.message });
+    });
+    req.end();
+  });
+}
+
+async function main() {
+  const urls = serviceUrls();
+  if (urls.length === 0) {
+    console.error("No service URLs found — is script.js available?");
+    process.exit(1);
+  }
+
+  const results = await Promise.all(urls.map(async (u) => [u, await checkUrl(u)]));
+  const services = Object.fromEntries(results);
+
+  const payload = { updatedAt: new Date().toISOString(), services };
+  fs.writeFileSync(OUT_FILE, JSON.stringify(payload, null, 2));
+
+  const count = (s) => results.filter(([, r]) => r.status === s).length;
+  console.log(`Health: ${urls.length} services — ${count("online")} online, ${count("error")} error, ${count("offline")} offline`);
+}
+
+main().catch((err) => {
+  console.error(err);
+  process.exit(1);
+});