feat: support choice+open dual question generation with judgment anchors

- Add judgment and followupHints fields to QuestionBankItem entity
- Rewrite generateQuestions prompt for 3:7 choice:open ratio
- Extract parseGeneratedQuestion function with type-aware parsing
- Add 29 unit tests: 14 prompt content + 15 parse logic
- Total: 97 tests passing (59 baseline + 38 new)

server/src/assessment/entities/question-bank-item.entity.ts

@@ -86,6 +86,12 @@ export class QuestionBankItem {
   @Column({ type: 'text', nullable: true })
   basis: string | null;
 
+  @Column({ type: 'text', nullable: true })
+  judgment: string | null;
+
+  @Column({ type: 'simple-json', nullable: true, name: 'followup_hints' })
+  followupHints: string[] | null;
+
   @Column({ name: 'created_by', nullable: true, type: 'text' })
   createdBy: string | null;