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/ai/services/usage-tracker.ts

@@ -0,0 +1,83 @@
+import "server-only"
+
+import { trackEvent, type EventName } from "@/shared/lib/track-event"
+
+export type AiUsageEvent = {
+  userId: string
+  capability: "chat" | "similar_question" | "grading_assist" | "lesson_content" | "question_variant" | "weakness_analysis"
+  providerId?: string
+  model?: string
+  success: boolean
+  durationMs: number
+  tokenUsage?: number
+  errorMessage?: string
+}
+
+const AI_EVENT_MAP: Record<AiUsageEvent["capability"], EventName> = {
+  chat: "ai.chat",
+  similar_question: "ai.similar_question",
+  grading_assist: "ai.grading_assist",
+  lesson_content: "ai.lesson_content",
+  question_variant: "ai.question_variant",
+  weakness_analysis: "ai.weakness_analysis",
+}
+
+/**
+ * AI 使用埋点
+ *
+ * 记录每次 AI 调用的元数据，用于监控、成本分析与异常排查。
+ * 非阻塞，失败不影响主流程。
+ */
+export const trackAiUsage = (event: AiUsageEvent): void => {
+  const eventName = AI_EVENT_MAP[event.capability]
+  void trackEvent({
+    event: eventName,
+    userId: event.userId,
+    targetType: event.capability,
+    properties: {
+      providerId: event.providerId,
+      model: event.model,
+      success: event.success,
+      durationMs: event.durationMs,
+      tokenUsage: event.tokenUsage,
+      errorMessage: event.errorMessage,
+    },
+  }).catch(() => {
+    // 静默失败：埋点不应影响业务流程
+  })
+}
+
+/**
+ * 测量 AI 调用耗时并自动埋点
+ */
+export const withAiTracking = async <T>(
+  userId: string,
+  capability: AiUsageEvent["capability"],
+  providerId: string | undefined,
+  fn: () => Promise<{ result: T; model?: string; tokenUsage?: number }>
+): Promise<T> => {
+  const start = Date.now()
+  try {
+    const { result, model, tokenUsage } = await fn()
+    trackAiUsage({
+      userId,
+      capability,
+      providerId,
+      model,
+      success: true,
+      durationMs: Date.now() - start,
+      tokenUsage,
+    })
+    return result
+  } catch (error) {
+    trackAiUsage({
+      userId,
+      capability,
+      providerId,
+      success: false,
+      durationMs: Date.now() - start,
+      errorMessage: error instanceof Error ? error.message : String(error),
+    })
+    throw error
+  }
+}