feat(textbooks): 知识图谱功能全面重构 — 前置依赖 + dagre 布局 + React Flow 可视化 + 师生双视角

将教材模块图谱从基本无用状态升级为完整知识图谱可视化系统。

数据层：新增 knowledgePointPrerequisites 表（复合主键+双外键 cascade）；新增 data-access-graph.ts（server-only）知识点关联聚合、学生/班级掌握度查询；utils.ts 新增 hasCycleAfterAddingEdge（DFS 循环依赖检测）。

业务层：3 个新 Server Action（getKnowledgeGraphDataAction 三视图模式、createPrerequisiteAction 含循环检测、deletePrerequisiteAction）；graph-layout.ts 重写为 dagre 分层有向图布局。

视图层：knowledge-graph.tsx 重写为 React Flow 主组件（全书视图+搜索高亮+关联节点高亮+章节着色）；4 个新组件（graph-kp-node/graph-prerequisite-edge/graph-toolbar/graph-node-detail-panel）；use-graph-data.ts 派生值模式避免 effect 中 setState。

架构：严格三层架构，客户端通过 Server Action 间接访问 server-only 数据层；权限校验+ i18n 全覆盖；架构文档 004/005 同步。

测试：utils.test.ts 新增 5 个循环检测测试，graph-layout.test.ts 重写 5 个 dagre 布局测试，全部 30 个教材模块单元测试通过。

附带提交 drizzle/0005 error-book 迁移文件以保持 journal 一致性。

drizzle/0004_calm_sandman.sql

@@ -0,0 +1,11 @@
+CREATE TABLE `knowledge_point_prerequisites` (
+	`knowledge_point_id` varchar(128) NOT NULL,
+	`prerequisite_kp_id` varchar(128) NOT NULL,
+	`created_at` timestamp NOT NULL DEFAULT (now()),
+	CONSTRAINT `knowledge_point_prerequisites_knowledge_point_id_prerequisite_kp_id_pk` PRIMARY KEY(`knowledge_point_id`,`prerequisite_kp_id`)
+);
+--> statement-breakpoint
+ALTER TABLE `knowledge_point_prerequisites` ADD CONSTRAINT `kp_prereq_kp_fk` FOREIGN KEY (`knowledge_point_id`) REFERENCES `knowledge_points`(`id`) ON DELETE cascade ON UPDATE no action;--> statement-breakpoint
+ALTER TABLE `knowledge_point_prerequisites` ADD CONSTRAINT `kp_prereq_prereq_fk` FOREIGN KEY (`prerequisite_kp_id`) REFERENCES `knowledge_points`(`id`) ON DELETE cascade ON UPDATE no action;--> statement-breakpoint
+CREATE INDEX `kp_prereq_kp_idx` ON `knowledge_point_prerequisites` (`knowledge_point_id`);--> statement-breakpoint
+CREATE INDEX `kp_prereq_prereq_idx` ON `knowledge_point_prerequisites` (`prerequisite_kp_id`);