feat(exam-homework): add audit report, i18n, error boundaries, and permission hardening

- Add comprehensive audit report for exam and homework module

- Create exam-homework i18n message files (zh-CN + en) and register namespace

- Add permission check to gradeHomeworkSubmissionAction to prevent horizontal privilege escalation

- Add Error Boundary + loading.tsx for 5 key pages (exam build/proctoring, homework assignment/submissions, student assignment)

- Refactor exam-columns to createExamColumns(t) factory for i18n support

- Refactor exam-data-table to manage columns internally via useTranslations

- Replace hardcoded strings with i18n keys in all exam/homework components and pages

- Add getHomeworkSubmissionForGrading data-access for secure grading flow

src/modules/homework/data-access-write.ts

@@ -45,7 +45,7 @@ export type HomeworkSubmissionPermissionData = {
 
 export type CreateHomeworkAssignmentData = {
   assignmentId: string
-  sourceExamId: string
+  sourceExamId: string | null
   title: string
   description: string | null
   structure: unknown
@@ -116,6 +116,32 @@ export const getHomeworkSubmissionForPermission = async (
   }
 }
 
+/**
+ * 批改权限校验：获取提交记录及其作业的创建者信息
+ * 用于 gradeHomeworkSubmissionAction 校验教师是否有权批改该提交
+ * 返回 null 表示提交记录不存在
+ */
+export const getHomeworkSubmissionForGrading = async (
+  submissionId: string
+): Promise<{
+  id: string
+  assignmentId: string
+  creatorId: string
+  sourceExamId: string | null
+} | null> => {
+  const submission = await db.query.homeworkSubmissions.findFirst({
+    where: eq(homeworkSubmissions.id, submissionId),
+    with: { assignment: true },
+  })
+  if (!submission) return null
+  return {
+    id: submission.id,
+    assignmentId: submission.assignmentId,
+    creatorId: submission.assignment.creatorId,
+    sourceExamId: submission.assignment.sourceExamId,
+  }
+}
+
 // ---- Write functions ----
 
 export const createHomeworkAssignment = async (