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/index.html

@@ -14,6 +14,11 @@
     <div class="topbar__meta">
       <span id="last-updated" class="muted" aria-live="polite">Loading…</span>
       <button id="refresh-btn" class="btn" type="button">Refresh</button>
+      <!-- Filled in from /api/v1/me when signed in with Microsoft 365 -->
+      <span id="signed-in" class="topbar__user" hidden>
+        <span id="signed-in-email" class="muted"></span>
+        <a id="sign-out" class="btn" href="#">Sign out</a>
+      </span>
     </div>
   </header>