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）

packages/shared-proto/proto/msg.proto

@@ -0,0 +1,48 @@
+syntax = "proto3";
+
+package next_edu_cloud.msg.v1;
+
+service NotificationService {
+  rpc SendNotification(SendNotificationRequest) returns (Notification);
+  rpc ListNotifications(ListNotificationsRequest) returns (ListNotificationsResponse);
+  rpc MarkAsRead(MarkAsReadRequest) returns (Empty);
+  rpc SearchNotifications(SearchNotificationsRequest) returns (SearchNotificationsResponse);
+}
+
+message Notification {
+  string id = 1;
+  string user_id = 2;
+  string type = 3;
+  string title = 4;
+  string content = 5;
+  string channel = 6;
+  bool is_read = 7;
+  int64 created_at = 8;
+}
+
+message SendNotificationRequest {
+  string user_id = 1;
+  string type = 2;
+  string title = 3;
+  string content = 4;
+  string channel = 5;
+}
+
+message ListNotificationsRequest {
+  string user_id = 1;
+  bool only_unread = 2;
+}
+
+message ListNotificationsResponse {
+  repeated Notification notifications = 1;
+}
+
+message MarkAsReadRequest { string id = 1; }
+message SearchNotificationsRequest {
+  string user_id = 1;
+  string query = 2;
+}
+message SearchNotificationsResponse {
+  repeated Notification notifications = 1;
+}
+message Empty {}