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。

src/modules/textbooks/components/graph-kp-node.tsx

@@ -0,0 +1,91 @@
+"use client"
+
+import { memo } from "react"
+import { Handle, Position, type NodeProps } from "@xyflow/react"
+import { useTranslations } from "next-intl"
+import { cn } from "@/shared/lib/utils"
+import type { GraphNodeData, MasteryLevel } from "../types"
+import type { GraphLayoutNodeData } from "../graph-layout"
+
+/** 根据掌握度计算色彩等级 */
+function getMasteryLevel(mastery: number | null): MasteryLevel {
+  if (mastery === null) return "unassessed"
+  if (mastery < 60) return "low"
+  if (mastery < 85) return "medium"
+  return "high"
+}
+
+const MASTERY_COLORS: Record<MasteryLevel, string> = {
+  low: "border-red-500 bg-red-50 dark:bg-red-950/30",
+  medium: "border-yellow-500 bg-yellow-50 dark:bg-yellow-950/30",
+  high: "border-green-500 bg-green-50 dark:bg-green-950/30",
+  unassessed: "border-border bg-card",
+}
+
+const MASTERY_BAR_COLORS: Record<MasteryLevel, string> = {
+  low: "bg-red-500",
+  medium: "bg-yellow-500",
+  high: "bg-green-500",
+  unassessed: "bg-muted",
+}
+
+function GraphKpNodeComponent({ data, selected }: NodeProps) {
+  const t = useTranslations("textbooks")
+  const nodeData = data as unknown as GraphLayoutNodeData
+  const { kp } = nodeData
+  const graphData = (data as unknown as { graphData?: GraphNodeData }).graphData
+  const mastery = graphData?.mastery ?? null
+  const masteryLevel = getMasteryLevel(mastery?.masteryLevel ?? null)
+  const showMastery = graphData?.viewMode === "student-mastery" || graphData?.viewMode === "class-mastery"
+
+  return (
+    <div
+      className={cn(
+        "rounded-lg border-2 px-3 py-2 shadow-sm transition-all",
+        MASTERY_COLORS[masteryLevel],
+        selected && "ring-2 ring-primary ring-offset-1",
+        graphData?.isHighlighted && "ring-2 ring-primary",
+        !graphData?.isHighlighted && graphData !== undefined && "opacity-40",
+      )}
+      style={{ width: 180 }}
+    >
+      <Handle type="target" position={Position.Top} className="opacity-0" />
+
+      <div className="flex items-start justify-between gap-2 mb-1">
+        <span className="text-xs font-medium line-clamp-2 flex-1">{kp.name}</span>
+        {kp.questionCount > 0 && (
+          <span className="text-[10px] px-1.5 py-0.5 rounded-full bg-primary/10 text-primary shrink-0">
+            {kp.questionCount} {t("graph.node.questions")}
+          </span>
+        )}
+      </div>
+
+      {showMastery && (
+        <div className="mt-2">
+          <div className="flex items-center justify-between text-[10px] text-muted-foreground mb-1">
+            <span>{t("graph.node.mastery")}</span>
+            <span>
+              {mastery ? `${Math.round(mastery.masteryLevel)}%` : t("graph.detail.masteryNotAssessed")}
+            </span>
+          </div>
+          <div className="h-1.5 rounded-full bg-muted overflow-hidden">
+            <div
+              className={cn("h-full transition-all", MASTERY_BAR_COLORS[masteryLevel])}
+              style={{ width: `${mastery?.masteryLevel ?? 0}%` }}
+            />
+          </div>
+        </div>
+      )}
+
+      {kp.chapterTitle && (
+        <div className="mt-1 text-[10px] text-muted-foreground truncate">
+          {kp.chapterTitle}
+        </div>
+      )}
+
+      <Handle type="source" position={Position.Bottom} className="opacity-0" />
+    </div>
+  )
+}
+
+export const GraphKpNode = memo(GraphKpNodeComponent)