docs(security): document the Codex sign-in boundary for OAuth voice

Add the Codex auth.json credential to the secrets table and describe the
loopback-only OAuth routes, the codex login launch and its limits.

SECURITY.md

@@ -18,6 +18,7 @@ The golden rule: **secret-bearing API keys stay on the server side.** The dev/pr
 | Key | Where it lives | How the browser uses it |
 |-----|----------------|--------------------------|
 | `OPENAI_API_KEY` | Server only | Browser fetches a short-lived **ephemeral** Realtime session token from `/api/realtime/token`; the real key never ships |
+| ChatGPT sign-in through Codex (optional, experimental) | Codex's own `auth.json` (`CODEX_HOME`, default `~/.codex`); GEV reads it server-side and never writes or refreshes it | Only after **USE CHATGPT OAUTH** is selected in Provider Settings: the server mints the same ephemeral Realtime token with the ChatGPT access token, for loopback clients only |
 | `AISSTREAM_API_KEY` | Server only | Server holds the AISStream websocket; browser polls the same-origin `/api/vessels` cache |
 | OpenSky OAuth (`OPENSKY_CLIENT_ID/SECRET`) | Server only | Server mints + refreshes the token behind `/api/flights` |
 | `GOOGLE_MAPS_SERVER_API_KEY` (optional, #33) | Server only | Server calls Places (`/api/google/nearby-places`, `/api/google/text-search`) and the Street View fallback with this key; falls back to `GOOGLE_MAPS_API_KEY` when unset |
@@ -74,6 +75,7 @@ The data proxies under `server/providers/` are written so the browser cannot tur
 - **Redacted debug logging.** The voice debug log (`.gev-logs/`, gitignored) strips API keys, bearer tokens, client secrets, and image data URLs before writing.
 - **Cross-site gate on cost/log endpoints.** The cost-bearing (`/api/realtime/token`, `/api/openai/hud-summary`, `/api/google/nearby-places`, `/api/google/text-search`), quota-bearing (`/api/mapillary/*`, with a per-IP limit on tiles) and log (`/api/realtime/debug-log`) endpoints refuse cross-site browser requests: a foreign or opaque `Origin`, a `Sec-Fetch-Site` other than `same-origin`/`none` (this is what blocks an `<img>`/navigation that carries no `Origin`), or any reverse-proxy/CDN forwarding header returns `403`. Non-browser loopback tools (the repo QA harnesses POST the token endpoint with no `Origin`) and the explicit `HOST=0.0.0.0` LAN opt-in keep working — the gate deliberately does not require a loopback remote address; that stricter requirement belongs only to the credential panel (`admitKeySetupRequest`). The Host check (`allowedHosts`, applied by `hostCheckPlugin` ahead of every route, since Vite's own check runs only after plugin middleware) rejects a rebinding `Host` before any handler runs, in every mode, and the gate is defense in depth behind it: the LAN opt-in keeps that check and accepts only IP addresses, `localhost` and names listed in `GEV_ALLOWED_HOSTS`. **Limit:** a name you list there is trusted like the app's own address, so list only names you control.
 - **Full Content-Security-Policy.** The dev and preview servers send a real CSP (`script-src 'self' 'unsafe-eval' blob:` plus the YouTube, Facebook and X scripts the Nepal event's embedded media loads, with a matching `frame-src` for their players and posts: no inline script and no other script origin; `blob:` covers the object URLs Cesium's bundled workers bootstrap through in the built app), alongside the existing `X-Frame-Options: DENY` and `frame-ancestors 'none'`. `'unsafe-eval'` stays because Knockout, bundled inside Cesium's widgets package, resolves the global object with `eval` at load time and the globe does not initialize without it; styles permit `'unsafe-inline'` and Google Fonts because Vite's dev client injects `<style>` elements.
+- **ChatGPT OAuth voice reuses the local Codex sign-in.** `/api/realtime/oauth-status`, `/api/realtime/oauth-login` and `/api/realtime/token?auth=oauth` answer only loopback connections, behind the same cross-site gate as the token endpoint; the login route also refuses a foreign `Origin`. None of them returns the access token. The login route runs `codex login` (or the executable named by `CODEX_BIN`) with an allowlisted environment that leaves out GEV's provider keys, keeps one attempt pending at a time and cancels it after two minutes or when the server stops. **Limits:** any program on this machine that can reach the server can mint Realtime sessions on the signed-in ChatGPT account, as it can with the API key; a local reverse proxy that hides the client's address defeats the loopback check; and OpenAI has not documented this use of Codex credentials for Realtime, so it may stop working or fall outside the account's terms.
 
 ## Network exposure — the operator threat model