Replace basic auth with Microsoft 365 sign-in

nginx now checks every dashboard and API request with oauth2-proxy
(auth_request), which signs users in with Microsoft Entra ID. Who may
sign in is controlled in Entra: a single-tenant app registration with
"Assignment required", so only assigned users/groups get through.

- deploy/oauth2-proxy.cfg (entra-id provider, auth-only mode, secure
  cookie, trusted proxy = localhost) and deploy/oauth2-proxy.service
- nginx: pages redirect to Microsoft sign-in and back to the same URL;
  API calls get a plain 401 (the page already asks to reload);
  /api/v1/ingest stays key-only for collectors; /oauth2/auth is
  internal; the signed-in user is passed to the API in headers nginx
  sets (so browsers can't spoof them); dashboard files are served with
  Cache-Control: no-cache so updates show up at once
- GET /api/v1/me; the top bar shows the signed-in user and a Sign out
  that also signs out of Microsoft (otherwise SSO signs straight back in)
- README: Entra and server setup steps

Tested locally with oauth2-proxy 7.15.5 and nginx 1.31 running the
deploy configs (TLS, self-signed): redirects to the tenant's sign-in
with the return URL kept, 401s, ingest bypass, sign-out allow-list,
cookie flags, security headers. The signed-in path was tested with a
stand-in for oauth2-proxy's auth answer; a real Entra sign-in wasn't.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

public/js/app.js

@@ -56,6 +56,9 @@ const dom = {
   empty: $('#empty-state'),
   search: $('#search'),
   refreshBtn: $('#refresh-btn'),
+  signedIn: $('#signed-in'),
+  signedInEmail: $('#signed-in-email'),
+  signOut: $('#sign-out'),
   lastUpdated: $('#last-updated'),
   errorBanner: $('#error-banner'),
   dialog: $('#detail-dialog'),
@@ -524,5 +527,20 @@ dom.search.addEventListener('input', () => {
 
 dom.refreshBtn.addEventListener('click', () => refresh().then(schedulePoll));
 
+// ---------- Signed-in user ----------
+async function showSignedInUser() {
+  try {
+    const me = await api.getMe();
+    const who = me?.email || me?.name;
+    if (!who) return;  // no Microsoft 365 sign-in in front of the dashboard (dev, mock)
+    dom.signedInEmail.textContent = who;
+    dom.signOut.href = CONFIG.signOutUrl;
+    dom.signedIn.hidden = false;
+  } catch (err) {
+    console.error(err);  // not worth a banner: the jobs request will surface real problems
+  }
+}
+
 // ---------- Boot ----------
+showSignedInUser();
 refresh().then(schedulePoll);