Add Bunny CDN purge retries and throttling

[davidmerfield]

Summary

• add retry and exponential backoff handling for Bunny CDN purge requests, including respect for Retry-After headers
• throttle purge requests to avoid hitting rate limits
• add tests covering retry, backoff, retry-after, and throttling scenarios

Testing

• NODE_PATH=. npx jasmine app/helper/tests/purgeCdnUrls.js

---

Codex Task

[davidmerfield]

@codex review

[chatgpt-codex-connector[bot]]

Codex Review: Didn't find any major issues. Chef's kiss.

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".