fix(gateway)!: replace token cookies with revocable sessions and harden abuse controls

- Replace user:token cookies with opaque server-side sessions, revocable on logout, delete, recreate and expiry; bind every credential to the path user and add cross-user regression tests.
- Rate-limit /users and /login, add optional signup invite codes and form same-origin checks, and unify login failures to 401.
- Require explicit ALLOW_INSECURE_COOKIES for plain HTTP, use __Host-dsh_gw_session when secure, and fix secure cookie deletion.
- Strip client-supplied forwarding/SSO headers, add an optional upstream allowlist, per-user in-flight and WebSocket message caps, credential-free handshake errors, and sweep-time state pruning.
- Digest-pin base images, hash-pin gateway dependencies, verify DSH_COMMIT in the build, and run the gateway image as non-root with a pinned Python base.
- Update manifests, production checklist, architecture/security docs and add an SSO preflight script.

BREAKING CHANGE: existing dsh_gw_session=<user>:<token> cookies are invalid; users must sign in again. Unknown users on POST /login now return 401 instead of 404, and the gateway refuses COOKIE_SECURE=false unless ALLOW_INSECURE_COOKIES=1.

examples/dsh-agents-as-a-service/manifests/optional/README.md

@@ -23,6 +23,12 @@ It requires a CNI that enforces `NetworkPolicy`. The selector uses the
 agent-sandbox controller's pod tracking label, so the policy applies to
 existing warm-pool and claimed sandboxes without recreation.
 
+**This is opt-in for the demo but required for production.** Without it every
+tenant can reach every other tenant and cluster-internal endpoints. Production
+also needs the gateway egress policy (cluster-specific API-server endpoint) and
+the exposure variant's policy; see the
+[production checklist](../../docs/optional-hardening.md#6-production-checklist).
+
 ## gVisor, nodeSelector and provider keys
 
 These are configured in the main manifests or with an extra Secret: