test(preprod): two-host routing simulation, drill suite, CI security scanning

- infra/scripts/simulate-two-host/: real two-Docker-network simulation
  (Gateway genuinely isolated from any Lab bridge network) proving the
  Lab Host Router fix - 13/13 real assertions (HTTP+WebSocket through the
  Router, Alice/Bob isolation, garbage/destroyed tokens, mTLS-only Router
  access).
- infra/scripts/drills/: Lab Host loss/rebuild (9/9), deployment rollback,
  CI-fails-closed (6 required failure modes), fake cohort dress rehearsal
  (14/14, one continuous narrative through the real app).
- infra/ci/gitea-workflows/build-and-test.yml: new security-scan job
  (npm audit, gitleaks, hadolint, shellcheck, trivy - all free/open-source),
  severity policy in docs/SECURITY_SCANNING.md (CRITICAL/HIGH blocks,
  MEDIUM/LOW reported only). Gates build-images alongside regression.

Found and fixed two real pre-existing bugs via the CI-fails-closed drill:
staging-smoke-test.sh and production-smoke-test.sh both had a classic
set -e gotcha (curl ... && echo "OK" as one statement - a failing command
that isn't last in an && chain doesn't trigger -e) that let an
unreachable/broken endpoint fall through silently instead of failing the
smoke test.

platform/test/e2e-phase3a.js updated for the new paginated
/api/admin/audit-events response shape (Phase 4.1 pagination work).

docs/SECURITY_SCANNING.md

@@ -0,0 +1,47 @@
+# Security Scanning (Phase 4.1 §24)
+
+`infra/ci/gitea-workflows/build-and-test.yml`'s `security-scan` job. All free, open-source,
+no account/signup required for any of them — no paid scanner was added.
+
+## What's scanned
+
+| Tool | Checks | Scope |
+|---|---|---|
+| `npm audit --audit-level=high` | Known-vulnerable dependencies | Every first-party Node service |
+| [gitleaks](https://github.com/gitleaks/gitleaks) | Committed secrets (keys, tokens, credentials) | Whole repo |
+| [hadolint](https://github.com/hadolint/hadolint) | Dockerfile best-practice/security lint | Every first-party Dockerfile |
+| [shellcheck](https://www.shellcheck.net/) | Shell script correctness/security | `infra/**/*.sh` |
+| [trivy](https://github.com/aquasecurity/trivy) | OS/library CVEs in a built container image | `platform-api` image (representative — same base/layers as the other Node services) |
+
+## Severity policy
+
+- **CRITICAL / HIGH** — fails the build (`security-scan` blocks `build-images`, which blocks
+  `deploy-staging`, which blocks production promotion — see the job dependency chain in the
+  workflow file).
+- **MEDIUM / LOW** — reported in the job output, never blocks. A build that's permanently red
+  over a low-severity transitive dependency finding nobody can immediately fix trains everyone
+  to ignore red builds, which is worse than not scanning at all.
+
+`npm audit --audit-level=high` and trivy's `--severity HIGH,CRITICAL` both implement this
+directly at the tool level, not via manual triage.
+
+## What's NOT in the automated job (and why)
+
+- **Dedicated IaC security linting** (checkov/tflint) — not added. `infra/scripts/validate-compose.sh`,
+  `infra/firewall/validate-firewall.sh`, `ansible-playbook --syntax-check`, and `tofu validate`
+  (all in the `regression` job) already catch structural/syntax problems in the infra code;
+  a dedicated security-policy linter for Terraform/Ansible is a reasonable Phase 5 addition once
+  there's a real cloud footprint to misconfigure, not before.
+- **DAST/penetration testing** — out of scope for CI; this is a build-time gate, not a running-system
+  test. Covered separately by `docs/PRODUCTION_VALIDATION_CHECKLIST.md`'s security section once
+  real hardware exists.
+- **SAST beyond hadolint/shellcheck** — the codebase is small enough (a handful of first-party
+  Node services) that `npm audit` + secret scanning + Dockerfile/shell lint cover the realistic
+  risk surface; a dedicated SAST tool (Semgrep, CodeQL) is a reasonable addition once the
+  codebase or team grows enough to justify the added CI time.
+
+## Local reproduction
+
+Every check above can be run locally with the exact same free tools — no CI-only dependency.
+See the `security-scan` job's steps in `infra/ci/gitea-workflows/build-and-test.yml` for the
+exact commands (they're copy-pasteable).