fix: harden auth against brute force, add auth audit logging, fix cookie/TLS gaps

Security review findings (HIGH-1/2/3, MEDIUM):

- Rate limiting via @nestjs/throttler: global 100 req/min default, stricter
  10 req/min on login and refresh. Also closes the MEDIUM finding (public
  batch-lookup enumeration) since it now sits behind the global limit too.
  Trust the K8s ingress's single proxy hop so throttling keys off the real
  client IP, not the ingress pod's IP.
- Auth events (login success/failure, logout, token refresh) are now
  audit-logged via new AuditAction values, closing the gap where the
  project's own audit-log principle never actually covered the Auth module.
- Refresh-token cookie now defaults to Secure=true (opt out only via
  COOKIE_SECURE=false for local HTTP dev), instead of keying off NODE_ENV
  which was never set in the K8s deploy templates. Added a TLS block +
  cert-manager annotation to both the backend and frontend ingress
  templates, which previously had no HTTPS termination configured at all.

deploy/k8s/backend/configmap.example.yaml

@@ -8,3 +8,4 @@ data:
   JWT_REFRESH_EXPIRES_IN: "7d"
   FRONTEND_ORIGIN: "https://apicore.example.com"
   DOCUMENTS_STORAGE_PATH: "/data/documents"
+  COOKIE_SECURE: "true"