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/data-ana/src/data_ana/clickhouse_client.py

@@ -0,0 +1,25 @@
+"""ClickHouse 客户端."""
+import clickhouse_connect
+from .config import settings
+
+_client = None
+
+
+def get_client():
+    """获取 ClickHouse 客户端."""
+    global _client
+    if _client is None:
+        _client = clickhouse_connect.get_client(
+            host=settings.clickhouse_host,
+            port=settings.clickhouse_port,
+            database=settings.clickhouse_database,
+        )
+    return _client
+
+
+async def close_client() -> None:
+    """关闭客户端."""
+    global _client
+    if _client:
+        _client.close()
+        _client = None