fix: shuffle bank questions + grader LLM error resilience

- selectQuestions: shuffle final result for random question order
- grader: wrap LLM invoke in try-catch, default score 5 on failure
- grader: inner try-catch for JSON parse errors, graceful fallback

server/src/assessment/services/question-bank.service.ts

@@ -588,7 +588,7 @@ export class QuestionBankService {
     this.logger.log(
       `[selectQuestions] Selected ${selected.length} questions from bank ${bankId}`,
     );
-    return selected;
+    return this.shuffleArray(selected);
   }
 
   async batchReviewItems(