feat(p5): messaging, push gateway and AI assistant services

P5 阶段交付物：
- services/msg: 消息通知服务（NestJS）
  - notifications: 发送通知 + ES 全文检索 + search
  - config/elasticsearch.ts: ES Client 单例
  - package.json: 补充 @opentelemetry/sdk-node + exporter-trace-otlp-http
- services/push-gateway: WebSocket 推送网关（Go Gin）
  - internal/hub/hub.go: WebSocket 连接池管理（Register/Unregister/SendToUser）
  - internal/ws/handler.go: JWT 鉴权 + WebSocket 升级 + 内部推送 API
- services/ai: AI 辅助服务（Python FastAPI）
  - /chat + /chat/stream（SSE 流式）
  - /generate/question + /optimize/expression
  - config.py: OpenAI 兼容 API 配置
- packages/shared-proto/proto/msg.proto: NotificationService 契约（send/search）
- packages/shared-proto/proto/ai.proto: AiService 契约（含 stream 方法）

services/msg/src/shared/observability/tracer.ts

@@ -0,0 +1,25 @@
+import { NodeSDK } from '@opentelemetry/sdk-node';
+import { OTLPTraceExporter } from '@opentelemetry/exporter-trace-otlp-http';
+import { env } from '../../config/env.js';
+
+let sdk: NodeSDK | null = null;
+
+export function initTracer(): void {
+  if (!env.OTEL_EXPORTER_OTLP_ENDPOINT) return;
+
+  sdk = new NodeSDK({
+    serviceName: 'msg',
+    traceExporter: new OTLPTraceExporter({
+      url: `${env.OTEL_EXPORTER_OTLP_ENDPOINT}/v1/traces`,
+    }),
+  });
+
+  sdk.start();
+  console.log('Tracer initialized');
+}
+
+export async function shutdownTracer(): Promise<void> {
+  if (sdk) {
+    await sdk.shutdown();
+  }
+}