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]>

backend/tests/test_api.py

@@ -122,3 +122,9 @@ def test_tenant_name_is_case_insensitive(client):
     body["tenant"] = "ACME CORP"
     client.post("/api/v1/ingest", json=body)
     assert len(client.get("/api/v1/jobs").json()["jobs"]) == 1
+
+
+def test_me_reports_signed_in_user_from_proxy_headers(client):
+    assert client.get("/api/v1/me").json() == {"email": None, "name": None}
+    r = client.get("/api/v1/me", headers={"X-Forwarded-Email": "[email protected]", "X-Forwarded-User": "abc-123"})
+    assert r.json() == {"email": "[email protected]", "name": "abc-123"}