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/main.ts

@@ -0,0 +1,31 @@
+import 'reflect-metadata';
+import { NestFactory } from '@nestjs/core';
+import { AppModule } from './app.module.js';
+import { GlobalErrorFilter } from './shared/errors/global-error.filter.js';
+import { initTracer, shutdownTracer } from './shared/observability/tracer.js';
+import { env } from './config/env.js';
+import { logger } from './shared/observability/logger.js';
+
+async function bootstrap(): Promise<void> {
+  initTracer();
+
+  const app = await NestFactory.create(AppModule, {
+    logger: ['log', 'error', 'warn'],
+  });
+
+  app.useGlobalFilters(new GlobalErrorFilter());
+  app.enableShutdownHooks();
+
+  await app.listen(env.PORT);
+  logger.info({ port: env.PORT }, 'Content service started');
+
+  process.on('SIGTERM', async () => {
+    await app.close();
+    await shutdownTracer();
+  });
+}
+
+bootstrap().catch((err: unknown) => {
+  logger.error({ err }, 'Failed to start content service');
+  process.exit(1);
+});