feat(content): graphql subgraph with dataloader for textbook/chapter/kp/question

- GraphQLModule: Apollo Federation 2 at /graphql

- Textbook/Chapter/KnowledgePoint/Question @key with @ResolveReference

- RouterAuthGuard: validate Router-Authorization header (ADR-036)

- batchFind methods added to 4 repositories

- Domain modules export repositories for GraphqlModule injection

services/content/src/graphql/resolvers/knowledge-point.resolver.ts

@@ -0,0 +1,79 @@
+/**
+ * content KnowledgePoint Resolver（v2.1 M1）
+ *
+ * Apollo Federation 子图：KnowledgePoint Entity
+ * - @key(fields: "id") 支持跨子图引用
+ * - @ResolveReference 使用 DataLoader 批量加载（ADR-035）
+ */
+import {
+  Resolver,
+  Query,
+  Args,
+  ID,
+  ResolveReference,
+  ObjectType,
+  Field,
+  Directive,
+} from "@nestjs/graphql";
+import {
+  DataLoaderService,
+  type KnowledgePointEntity,
+} from "../dataloader.service.js";
+
+/**
+ * KnowledgePoint ObjectType（Federation @key）
+ * 对应 content_knowledge_points 表
+ */
+@ObjectType()
+@Directive(`@key(fields: "id")`)
+export class KnowledgePoint {
+  @Field(() => ID)
+  id!: string;
+
+  @Field()
+  chapterId!: string;
+
+  @Field()
+  title!: string;
+
+  @Field({ nullable: true })
+  description: string | null = null;
+
+  @Field()
+  difficulty!: number;
+
+  @Field()
+  createdAt!: Date;
+
+  @Field()
+  updatedAt!: Date;
+}
+
+@Resolver(() => KnowledgePoint)
+export class KnowledgePointResolver {
+  constructor(private readonly loader: DataLoaderService) {}
+
+  /**
+   * Federation Reference Resolver
+   *
+   * 当其他子图通过 @key 引用 KnowledgePoint 时，Router 调用此方法解析。
+   * 使用 DataLoader 批量加载，消除 N+1 查询（ADR-035）。
+   */
+  @ResolveReference()
+  async resolveReference(ref: {
+    id: string;
+  }): Promise<KnowledgePointEntity | null> {
+    return this.loader.knowledgePointLoader.load(ref.id);
+  }
+
+  /**
+   * Query: knowledgePoint(id) → KnowledgePoint
+   * 通过 Apollo Router 访问，直连被 RouterAuthGuard 拒绝（ADR-036）
+   */
+  @Query(() => KnowledgePoint, { nullable: true })
+  async knowledgePoint(
+    @Args("id", { type: () => ID }) id: string,
+  ): Promise<KnowledgePointEntity | null> {
+    return this.loader.knowledgePointLoader.load(id);
+  }
+}