Explore implementing exponential backoff for HQ TTS downloads

[Amnish04]

On non-enterprise accounts, the limit for tts-1-hd model is just 3 RPM, which makes it impossible for people to download longer messages as audio with lower RPM keys.

As discussed in https://github.com/tarasglek/chatcraft.org/pull/627#issuecomment-2095352160,

we could implement exponential backoff strategy to limit the rate at which TTS download requests are sent if we start getting rate limit errors.

This library could be of help, but need to research more. https://www.npmjs.com/package/exponential-backoff

We could also have a setting, so user can choose between tts-1 (faster downloads) or tts-1-hd (high quality) as some downloads could be extremely slow with 3 RPM.