fix: add custom domain CORS support for plyr.fm (#79)
* docs: add custom domain (plyr.fm) to deployment documentation
- updated CORS configuration to support plyr.fm and www.plyr.fm
- documented FRONTEND_CORS_ORIGIN_REGEX in production secrets
- staging doesn't need custom CORS since it only uses cloudflare preview URLs
* docs: show exact production CORS regex (without localhost)
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