[Incident][108dev] Questions submission 500 on /api/questions/grader/interaction

## Summary
Frontend questions page fails when submitting an answer.

Observed error:
- POST /api/questions/grader/interaction -> 500 Internal Server Error
- Console surfaced AxiosError during answer submission.

## Root Cause
case_id selected in UI may reference an assembly without graders (or unresolved assembly), which currently raises a ValueError inside questions orchestration and bubbles to 500.

## Fix implemented (code)
- Added resilient fallback in questions API endpoint:
- catches ValueError/RuntimeError from evaluation orchestration
- returns HTTP 200 with baseline feedback payload in QuestionEvaluationResult shape
- prevents frontend hard failure while Foundry assembly provisioning is stabilized.

## Deployment
Break-glass direct live deployment will update utor-questions-108dev with this fix.

Related: #139 #141