feat(teacher): 题库模块（QuestionBank）

工作内容

- 新增 /teacher/questions 页面，支持 Suspense/空状态/加载态

- 题库 CRUD Server Actions：创建/更新/递归删除子题，变更后 revalidatePath

- getQuestions 支持 q/type/difficulty/knowledgePointId 筛选与分页返回 meta

- UI：表格列/筛选器/创建编辑弹窗，content JSON 兼容组卷

- 更新中文设计文档：docs/design/004_question_bank_implementation.md

src/app/(dashboard)/teacher/exams/[id]/build/page.tsx

@@ -42,11 +42,7 @@ export default async function BuildExamPage({ params }: { params: Promise<{ id:
           image: q.author.image || null,
         }
       : null,
-    knowledgePoints:
-      q.questionsToKnowledgePoints?.map((kp) => ({
-        id: kp.knowledgePoint.id,
-        name: kp.knowledgePoint.name,
-      })) ?? [],
+    knowledgePoints: q.knowledgePoints ?? [],
   })
 
   const questionOptionsById = new Map<string, Question>()