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

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Codex Task

[davidmerfield]

@codex review

[chatgpt-codex-connector]

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

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