fix: add status guards to prevent data loss

- create: auto-delete REJECTED→throw error; add tenantId filter
- remove: forbid PUBLISHED bank deletion
- removeItem: forbid PUBLISHED item deletion
- generateQuestions: restrict to DRAFT status only
- frontend: render MULTIPLE_CHOICE options/judgment/followupHints
- frontend: add judgment and followupHints to QuestionBankItem type
- add 12 service guard tests (109 total)

web/services/questionBankService.ts

@@ -17,6 +17,8 @@ export interface QuestionBankItem {
   questionType: 'SHORT_ANSWER' | 'MULTIPLE_CHOICE' | 'TRUE_FALSE';
   options?: string[] | null;
   correctAnswer?: string | null;
+  judgment?: string | null;
+  followupHints?: string[] | null;
   keyPoints: string[];
   difficulty: 'STANDARD' | 'ADVANCED' | 'SPECIALIST';
   dimension: 'PROMPT' | 'LLM' | 'IDE' | 'DEV_PATTERN' | 'WORK_CAPABILITY';