Make AI Coach message length configurable in Admin Advanced settings

[SSamDav]

Problem

AI Coach chat/refinement and review-note input is currently hardcoded to 1,000 characters in both the client and API. This prevents longer coaching context from being submitted and causes the current use case to fail.

Requested change

Add an Admin → AI Coach → Advanced setting for the maximum user message length.

The configured value should be enforced consistently by the chat UI and API routes (/api/coach/plan and /api/coach/review), rather than relying on the current hardcoded 1,000-character limit.

Acceptance criteria

• An admin can view and change the maximum AI Coach input length in Advanced settings.
• The value persists with the existing Coach configuration.
• The chat UI uses the configured limit for its input maxLength.
• The server validates or truncates refinement and review-note input with the same configured limit.
• A safe default remains 1,000 characters for existing instances.
• Include boundary coverage for the configured limit.

[DuarteSantos8]

Done in v1.3.9 via #280 (thanks @bluzername): Admin → Coach → Advanced sets the maximum message length (200–4000), and the chat and the API both use it. Thanks @SSamDav for filing this and for #268.

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