feat(ai): 新增 AI 模块并集成至备课/错题集/试卷/改题四大业务场景

- 新增 src/modules/ai 独立模块，遵循三层架构（actions → services → shared/lib/ai）
- 通过 AiClientProvider + useAiClient 实现 React Context 依赖注入，业务组件零直接 import
- 6 个 Server Actions 均调用 requirePermission() 权限校验，返回 ActionState<T>
- withAiTracking 统一埋点，覆盖 chat/similar_question/grading_assist/lesson_content/question_variant/weakness_analysis
- 集成场景：作业批改 AiGradingAssist、错题集 AiErrorBookAnalysis、备课 AiLessonContentGenerator、试卷 AiQuestionVariantGenerator
- 全量 i18n（en/zh-CN ai.json），Error Boundary + Skeleton 边界处理
- 同步架构图 004/005，新增审计报告 ai-module-audit-report.md

src/modules/homework/components/homework-grading-view.tsx

@@ -27,6 +27,7 @@ import { Tooltip, TooltipContent, TooltipTrigger } from "@/shared/components/ui/
 import { gradeHomeworkSubmissionAction } from "../actions"
 import { formatDate } from "@/shared/lib/utils"
 import { QuestionRenderer } from "./question-renderer"
+import { AiGradingAssist } from "@/modules/ai/components/ai-grading-assist"
 import {
   applyAutoGrades as applyAutoGradesUtil,
   extractAnswerValue,
@@ -136,16 +137,21 @@ export function HomeworkGradingView({
     formData.set("submissionId", submissionId)
     formData.set("answersJson", JSON.stringify(payload))
 
-    const result = await gradeHomeworkSubmissionAction(null, formData)
+    try {
+      const result = await gradeHomeworkSubmissionAction(null, formData)
 
-    if (result.success) {
-      toast.success(t("homework.grade.gradesSaved"))
-      // Optionally redirect or stay
-      router.refresh()
-    } else {
-      toast.error(result.message || t("homework.grade.gradesSaveFailed"))
+      if (result.success) {
+        toast.success(t("homework.grade.gradesSaved"))
+        // Optionally redirect or stay
+        router.refresh()
+      } else {
+        toast.error(result.message || t("homework.grade.gradesSaveFailed"))
+      }
+    } catch {
+      toast.error(t("homework.grade.gradesSaveFailed"))
+    } finally {
+      setIsSubmitting(false)
     }
-    setIsSubmitting(false)
   }
 
   const handleScrollToQuestion = (id: string) => {
@@ -337,6 +343,22 @@ export function HomeworkGradingView({
                          />
                        </div>
                      )}
+
+                     {/* AI Grading Assist (subjective questions only) */}
+                     {!isAutoGradable(ans) && (
+                       <AiGradingAssist
+                         questionText={extractQuestionText(ans.questionContent)}
+                         questionType={ans.questionType}
+                         studentAnswer={formatStudentAnswer(ans.studentAnswer)}
+                         correctAnswer={getTextCorrectAnswers(ans.questionContent).join(" / ")}
+                         maxScore={ans.maxScore}
+                         onApplyScore={(score) => handleManualScoreChange(ans.id, String(score))}
+                         onApplyFeedback={(feedback) => {
+                           handleFeedbackChange(ans.id, feedback)
+                           setShowFeedbackByAnswerId((prev) => ({ ...prev, [ans.id]: true }))
+                         }}
+                       />
+                     )}
                   </CardFooter>
                 </Card>
               )
@@ -520,3 +542,21 @@ const formatStudentAnswer = (studentAnswer: unknown): string => {
   if (v == null) return "—"
   return JSON.stringify(v)
 }
+
+/**
+ * 从题目内容中提取纯文本（用于 AI 批改输入）
+ *
+ * 优先使用 `text` 字段；若不存在则回退到 JSON 字符串，
+ * 保证 AI 服务能拿到可读的题目描述。
+ */
+const extractQuestionText = (content: QuestionContent | null): string => {
+  if (!content) return ""
+  if (typeof content.text === "string" && content.text.trim().length > 0) {
+    return content.text
+  }
+  try {
+    return JSON.stringify(content)
+  } catch {
+    return ""
+  }
+}