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/exposure/gateway-api-sso/75-networkpolicies.yaml

@@ -1,5 +1,5 @@
-# OPTIONAL with the SSO exposure variant: enforce the trust boundary at the
-# network layer.
+# REQUIRED with the SSO exposure variant: enforce the trust boundary at the
+# network layer. Applying the SSO variant without this file is demo-only.
 #
 # The chain is Traefik (Gateway API) -> oauth2-proxy -> dsh-gateway. These
 # policies make it the ONLY chain:
@@ -8,8 +8,9 @@
 #   - oauth2-proxy egress is limited to DNS, public HTTPS (GitHub) and the
 #     gateway.
 # The gateway's own egress (Kubernetes API, sandbox pods) is intentionally not
-# restricted here: the API-server endpoint is cluster-specific. See
-# ../../../docs/optional-hardening.md for that recipe.
+# restricted here: the API-server endpoint is cluster-specific. Add it for
+# production. See
+# ../../../docs/optional-hardening.md#gateway-egress-required-for-production.
 #
 # This manifest assumes Traefik, matching gatewayClassName: traefik in
 # 70-gateway.yaml. Adjust the app.kubernetes.io/name selector for another