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/shared/observability/metrics.ts

@@ -0,0 +1,24 @@
+import promClient from 'prom-client';
+
+const registry = new promClient.Registry();
+// 修复：在本地 registry 上设置默认标签（原代码误用全局 register）
+registry.setDefaultLabels({ service: 'content' });
+
+registry.registerMetric(
+  new promClient.Counter({
+    name: 'content_requests_total',
+    help: 'Total number of content requests',
+    labelNames: ['method', 'endpoint', 'status'],
+  }),
+);
+
+registry.registerMetric(
+  new promClient.Histogram({
+    name: 'content_request_duration_seconds',
+    help: 'Content request duration in seconds',
+    labelNames: ['method', 'endpoint'],
+    buckets: [0.01, 0.05, 0.1, 0.3, 0.5, 1, 3, 5],
+  }),
+);
+
+export { registry as metricsRegistry };