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/push-gateway/main.go

@@ -0,0 +1,63 @@
+package main
+
+import (
+	"context"
+	"log"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"github.com/edu-cloud/push-gateway/internal/config"
+	"github.com/edu-cloud/push-gateway/internal/hub"
+	"github.com/edu-cloud/push-gateway/internal/ws"
+	"github.com/gin-gonic/gin"
+)
+
+func main() {
+	cfg := config.Load()
+	gin.SetMode(gin.ReleaseMode)
+
+	h := hub.NewHub()
+	wsHandler := ws.NewHandler(h, cfg.JWTSecret)
+
+	r := gin.New()
+	r.Use(gin.Recovery())
+
+	r.GET("/healthz", func(c *gin.Context) {
+		c.JSON(200, gin.H{"status": "ok", "service": "push-gateway"})
+	})
+
+	// WebSocket 升级端点
+	r.GET("/ws", wsHandler.HandleWebSocket)
+
+	// 内部推送 API（Msg 服务调用）
+	api := r.Group("/internal")
+	api.POST("/push", wsHandler.PushHandler)
+
+	srv := &http.Server{
+		Addr:         ":" + cfg.Port,
+		Handler:      r,
+		ReadTimeout:  10 * time.Second,
+		WriteTimeout: 60 * time.Second, // WebSocket 长连接
+	}
+
+	go func() {
+		log.Printf("Push Gateway listening on :%s", cfg.Port)
+		if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+			log.Fatalf("listen: %s\n", err)
+		}
+	}()
+
+	quit := make(chan os.Signal, 1)
+	signal.Notify(quit, syscall.SIGINT, syscall.SIGTERM)
+	<-quit
+	log.Println("Shutting down Push Gateway...")
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	if err := srv.Shutdown(ctx); err != nil {
+		log.Fatal("Server forced to shutdown:", err)
+	}
+}