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/ingress/61-networkpolicy.yaml

@@ -1,11 +1,16 @@
-# OPTIONAL with the Ingress exposure variant: restrict the gateway's ingress
-# to the ingress controller.
+# REQUIRED with the Ingress exposure variant for production: restrict the
+# gateway's ingress to the ingress controller. Applying the variant without
+# this policy is demo-only.
 #
 # This manifest assumes Traefik, matching this variant's
 # `ingressClassName: traefik`. Traefik's Helm chart labels its pods
 # app.kubernetes.io/name=traefik; adjust the selector for another controller or
 # for a hostNetwork install (where the source is the node IP, not a pod IP).
 #
+# The sandbox NetworkPolicy and the gateway egress policy are separate and also
+# required for production; see
+# ../../../docs/optional-hardening.md#6-production-checklist.
+#
 # NetworkPolicy enforcement of kubelet probe traffic is CNI-dependent. If the
 # gateway starts failing its /healthz probes, allow the node CIDR on port 8080
 # or remove this policy. See ../../../docs/optional-hardening.md.