Add edge rate limiting templates (nginx + ALB/WAF)

[mikemcdougall]

Context

In-app rate limiting has been removed; rate limiting is enforced at the edge.

Tasks

• Add an nginx rate limiting example (per-IP) with standard headers and forwarded header handling.
• Add an AWS ALB + WAF rate-based rule example or Terraform snippet for rate limiting.
• Document where these templates live and how to enable them alongside ForwardedHeaders/Public BaseUrl.

Acceptance

• Sample configs checked into the repo (deploy/ or docs/examples).
• Docs reference the templates and clarify edge enforcement.