feat(p4): content analysis service with Neo4j knowledge graph and ClickHouse analytics

P4 阶段交付物：
- services/content: 内容资源服务（NestJS）
  - textbooks: 教材 CRUD + 知识图谱绑定
  - config/neo4j.ts: Neo4j driver 单例
  - textbooks.service.ts: MySQL CRUD + Neo4j 知识图谱（createKnowledgeGraph/getPrerequisites）
  - package.json: 补充 @opentelemetry/sdk-node + exporter-trace-otlp-http
- services/data-ana: 数据分析服务（Python FastAPI）
  - main.py: FastAPI + /healthz + class_performance + student_weakness 骨架
  - clickhouse_client.py: ClickHouse 客户端封装
  - config.py: 环境变量配置
- packages/shared-proto/proto/content.proto: TextbookService + KnowledgeGraphService 契约
- packages/shared-proto/proto/analytics.proto: AnalyticsService 契约（class_performance/student_weakness）

services/content/src/config/neo4j.ts

@@ -0,0 +1,11 @@
+import neo4j from 'neo4j-driver';
+import { env } from './env.js';
+
+export const neo4jDriver = neo4j.driver(
+  env.NEO4J_URL,
+  neo4j.auth.basic('neo4j', env.NEO4J_PASSWORD)
+);
+
+export async function closeNeo4j(): Promise<void> {
+  await neo4jDriver.close();
+}