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/notifications/notifications.controller.ts

@@ -0,0 +1,32 @@
+import { Body, Controller, Get, Param, Post, Query, Req } from '@nestjs/common';
+import { NotificationsService } from './notifications.service.js';
+import type { SendNotificationDto } from './notifications.service.js';
+
+@Controller('notifications')
+export class NotificationsController {
+  constructor(private readonly service: NotificationsService) {}
+
+  @Post()
+  async send(@Body() body: unknown) {
+    const result = await this.service.send(body as SendNotificationDto);
+    return { success: true, data: result };
+  }
+
+  @Get()
+  async list(@Req() req: { userId: string }, @Query('unread') unread: string) {
+    const result = await this.service.listByUser(req.userId, unread === 'true');
+    return { success: true, data: result };
+  }
+
+  @Post(':id/read')
+  async markAsRead(@Param('id') id: string) {
+    await this.service.markAsRead(id);
+    return { success: true };
+  }
+
+  @Get('search')
+  async search(@Req() req: { userId: string }, @Query('q') q: string) {
+    const result = await this.service.search(req.userId, q);
+    return { success: true, data: result };
+  }
+}