feat(lesson-preparation): 备课模块审计重构 — 跨模块解耦 + i18n + 纯函数抽取 + 错误边界

P0-1 跨模块直查修复：publish-service 不再直查 examQuestions 表，新增 exams/data-access.addExamQuestions 接口，复用 classes/data-access.getStudentIdsByClassIds

P0-2 i18n 接入：新增 zh-CN/en 翻译文件，注册 lessonPreparation 命名空间，17 个组件改造为 useTranslations/getTranslations

P1 纯函数抽取：lib/document-migration.ts（类型守卫替代 as 断言）、lib/node-summary.ts（翻译函数注入）、lib/rf-mappers.ts

P1 错误边界+骨架屏：新增 LessonPlanErrorBoundary 和 4 个 Skeleton 组件

P1 Block 注册表：新增 config/block-registry.tsx（BlockRenderer 组件），node-edit-panel 重构为配置驱动渲染

P1 其他修复：exercise-block 改用 router.refresh()，node-editor/lesson-node 复用 lib/ 纯函数

架构图同步：更新 004 和 005 文档

Refs: docs/architecture/audit/lesson-preparation-audit-report.md

src/modules/lesson-preparation/lib/rf-mappers.ts

@@ -0,0 +1,43 @@
+import type { Node, Edge } from "@xyflow/react";
+import type { LessonPlanNode, LessonPlanEdge } from "../types";
+
+/**
+ * 纯函数：将课案 nodes/edges 映射为 React Flow 格式。
+ * 从 node-editor.tsx 抽取，便于单元测试。
+ */
+
+export function toRfNodes(
+  nodes: LessonPlanNode[],
+  selectedNodeId: string | null,
+): Node[] {
+  return nodes.map((n) => ({
+    id: n.id,
+    type: "lesson",
+    position: n.position,
+    data: { node: n } as Record<string, unknown>,
+    selected: n.id === selectedNodeId,
+  }));
+}
+
+export function toRfEdges(edges: LessonPlanEdge[]): Edge[] {
+  return edges.map((e) => ({
+    ...e,
+    animated: true,
+    style: { stroke: "#1976d2", strokeWidth: 2 },
+  }));
+}
+
+/**
+ * 将 React Flow edges 转回课案 edges 格式。
+ */
+export function fromRfEdges(
+  rfEdges: Edge[],
+): LessonPlanEdge[] {
+  return rfEdges.map((e) => ({
+    id: e.id,
+    source: e.source,
+    target: e.target,
+    sourceHandle: e.sourceHandle ?? null,
+    targetHandle: e.targetHandle ?? null,
+  }));
+}