feat: add server-side version update check

Logs current version on startup and checks GitHub for newer releases.
Only runs in production, output stays in server logs (no client exposure).

instrumentation.ts

@@ -0,0 +1,43 @@
+import { readFileSync } from "fs";
+
+const VERSION_CHECK_URL =
+  "https://raw.githubusercontent.com/root-fr/jmap-webmail/main/VERSION";
+
+const SEMVER_RE = /^\d+\.\d+\.\d+$/;
+
+function compareVersions(current: string, remote: string): number {
+  const a = current.split(".").map(Number);
+  const b = remote.split(".").map(Number);
+  for (let i = 0; i < 3; i++) {
+    if ((b[i] ?? 0) > (a[i] ?? 0)) return 1;
+    if ((b[i] ?? 0) < (a[i] ?? 0)) return -1;
+  }
+  return 0;
+}
+
+export async function register() {
+  const pkg = JSON.parse(
+    readFileSync(`${process.cwd()}/package.json`, "utf-8")
+  );
+  const current: string = pkg.version ?? "0.0.0";
+  console.info(`JMAP Webmail v${current}`);
+
+  if (process.env.NODE_ENV !== "production") return;
+
+  try {
+    const res = await fetch(VERSION_CHECK_URL, {
+      cache: "no-store",
+      signal: AbortSignal.timeout(5000),
+    });
+    if (!res.ok) return;
+    const remote = (await res.text()).trim();
+    if (!SEMVER_RE.test(remote)) return;
+    if (compareVersions(current, remote) > 0) {
+      console.info(
+        `Update available: v${remote} — https://github.com/root-fr/jmap-webmail`
+      );
+    }
+  } catch {
+    // Network unavailable — not critical
+  }
+}