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/textbooks/textbooks.module.ts

@@ -1,12 +1,13 @@
 import { Module } from "@nestjs/common";
 import { TextbooksController } from "./textbooks.controller.js";
 import { TextbooksService } from "./textbooks.service.js";
+import { TextbooksRepository } from "./textbooks.repository.js";
 import { OutboxModule } from "../shared/outbox/outbox.module.js";
 
 @Module({
   imports: [OutboxModule],
   controllers: [TextbooksController],
-  providers: [TextbooksService],
-  exports: [TextbooksService],
+  providers: [TextbooksService, TextbooksRepository],
+  exports: [TextbooksService, TextbooksRepository],
 })
 export class TextbooksModule {}