[AGE-2291] LLM as Judge: No clear error message when API key is missing

[mmabrouk]

When users don't have an LLM API key in their settings and try to use LLM as a judge, they get a cryptic 500 error with no explanation. We should provide a clear, user-friendly error message explaining they need to add an API key to use this feature.

_{From SyncLinear.com | AGE-2291}