Add gateway README

gateway/README.md

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+# gateway
+
+A FastAPI app on Modal that puts scale-to-zero demos behind signed invite links. An invite link
+starts a session cookie that lasts up to 8 hours, and the session unlocks `/menu`.
+
+- `app.py`: the web app (`/i/{token}` to redeem an invite, `/menu`, `/healthz`)
+- `auth.py`: signs and checks invite tokens and session cookies
+- `state.py`: invite records stored in the `gateway-state` Modal Dict
+- `invites.py`: local CLI to create, list, and revoke invites
+
+## Setup
+
+The web app and the CLI must use the same `SIGNING_KEY`: the app reads it from the `gateway`
+Modal Secret, and the CLI reads it from a local `.env`.
+
+```sh
+KEY=$(python -c "import secrets; print(secrets.token_urlsafe(32))")
+modal secret create gateway SIGNING_KEY=$KEY
+echo "SIGNING_KEY=$KEY" > .env
+```
+
+## Deploy
+
+Pushes to `main` that touch `gateway/` are deployed by `.github/workflows/gateway.yml`. To deploy
+by hand:
+
+```sh
+uv run modal deploy app.py
+```
+
+## Invites
+
+```sh
+uv run modal run invites.py::create --label <name> --days 1 --budget-s 60  # prints the link
+uv run modal run invites.py::list
+uv run modal run invites.py::revoke --iid <iid>
+```
+
+## Checks
+
+These are the checks CI runs:
+
+```sh
+uv run ruff check . && uv run ruff format --check . && uv run mypy . && uv run pytest tests -q
+```