feat(e2e): deployed-environment E2E harness + gated test-auth (v0.52.0)

The §9 pipeline's PPE+E2E stage was unreachable: e2e/metadata.spec.js was
bound to Tier-1-only scaffolding (docker-seeded faceted collection,
SQLite-injected owner, Mailpit OTC sink). This makes the same suite run
against a deployed host.

- backend: POST /auth/test/login — fail-closed, secret-gated, single-
  identity owner test-login (404 unless E2E_TEST_AUTH_SECRET +
  E2E_TEST_AUTH_EMAIL both set; constant-time compare; loud startup warn).
  6 vertical tests; backend 665 green. Documented in backend/.env.example.
- e2e/lib/auth.js: branch on E2E_TEST_AUTH_SECRET (deployed test-login vs
  local Mailpit OTC); OWNER_EMAIL from E2E_OWNER_EMAIL. Spec unchanged so
  the localhost Tier-1 path keeps working.
- testing/seed-ppe.sh: seed a dedicated, prod-untouching PPE registry +
  content repo (faceted bdd collection) — real OHM content never touched.
- docs/design/2026-06-07-deployed-env-e2e-harness.md; CHANGELOG; VERSION
  + frontend/package.json -> 0.52.0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "rfc-app-frontend",
   "private": true,
-  "version": "0.51.1",
+  "version": "0.52.0",
   "type": "module",
   "scripts": {
     "dev": "vite",