Make AI Coach message length configurable

[SSamDav]

Closes #267

Summary

• add a persisted Admin → AI Coach → Advanced message-length setting
• enforce the configured 100–10,000 character range in the Coach chat and API routes
• retain the existing 1,000-character default for current instances

Verification

• node --test test/routes.test.js
• npm test in frontend/
• npm run build in frontend/

[DuarteSantos8]

Thanks @SSamDav, both for #267 and this PR. I went with #280: it also raises the limit inside the Coach's prompt builder (otherwise a longer message is still cut to 1,000 there) and doesn't add the unrelated output-token setting. The configurable length is in v1.3.9, so I'm closing this as a duplicate.

Released in v1.3.9: https://github.com/DuarteSantos8/openGym/releases/tag/v1.3.9