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/app/main.py

@@ -25,6 +25,16 @@ def health(conn: sqlite3.Connection = Depends(db.get_db)):
     return {"status": "ok"}
 
 
+@router.get("/me")
+def me(request: Request):
+    # Set by nginx from oauth2-proxy's sign-in check (nginx overwrites anything the browser sends).
+    # Display only: access control happens in nginx. Empty in dev or without Microsoft sign-in.
+    return {
+        "email": request.headers.get("x-forwarded-email") or None,
+        "name": request.headers.get("x-forwarded-preferred-username") or request.headers.get("x-forwarded-user") or None,
+    }
+
+
 @router.get("/jobs")
 def list_jobs(conn: sqlite3.Connection = Depends(db.get_db)):
     return {"generated_at": to_db(utcnow()), "jobs": repo.list_jobs(conn)}