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/ai.proto

@@ -0,0 +1,60 @@
+syntax = "proto3";
+
+package next_edu_cloud.ai.v1;
+
+service AiService {
+  rpc Chat(ChatRequest) returns (ChatResponse);
+  rpc StreamChat(ChatRequest) returns (stream ChatChunk);
+  rpc GenerateQuestion(GenerateQuestionRequest) returns (GeneratedQuestion);
+  rpc OptimizeExpression(OptimizeExpressionRequest) returns (OptimizedExpression);
+}
+
+message ChatRequest {
+  repeated ChatMessage messages = 1;
+  string model = 2;
+  double temperature = 3;
+}
+
+message ChatMessage {
+  string role = 1;
+  string content = 2;
+}
+
+message ChatResponse {
+  string content = 1;
+  string model = 2;
+  Usage usage = 3;
+}
+
+message Usage {
+  int32 prompt_tokens = 1;
+  int32 completion_tokens = 2;
+  int32 total_tokens = 3;
+}
+
+message ChatChunk {
+  string content = 1;
+  bool done = 2;
+}
+
+message GenerateQuestionRequest {
+  string prompt = 1;
+  string subject = 2;
+  string difficulty = 3;
+}
+
+message GeneratedQuestion {
+  string question = 1;
+  string answer = 2;
+  string explanation = 3;
+}
+
+message OptimizeExpressionRequest {
+  string text = 1;
+  string context = 2;
+}
+
+message OptimizedExpression {
+  string optimized = 1;
+  repeated string suggestions = 2;
+}