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.

.github/workflows/ci-python.yaml

@@ -43,7 +43,9 @@ jobs:
         run: |
           set -euo pipefail
           python -m pip install --upgrade pip
-          pip install -r requirements.txt -r requirements-dev.txt
+          # Both files are fully hash-pinned (uv pip compile --generate-hashes);
+          # pip refuses to install anything not listed with a matching hash.
+          pip install --require-hashes -r requirements.txt -r requirements-dev.txt
       - name: Lint
         run: |
           set -euo pipefail