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/config/database.ts

@@ -0,0 +1,24 @@
+import { drizzle } from 'drizzle-orm/mysql2';
+import mysql from 'mysql2/promise';
+import { env } from './env.js';
+
+let pool: mysql.Pool | null = null;
+
+export function getDb() {
+  if (!pool) {
+    pool = mysql.createPool({
+      uri: env.DATABASE_URL,
+      waitForConnections: true,
+      connectionLimit: 10,
+      queueLimit: 0,
+    });
+  }
+  return drizzle(pool);
+}
+
+export async function closeDb(): Promise<void> {
+  if (pool) {
+    await pool.end();
+    pool = null;
+  }
+}

services/msg/src/config/elasticsearch.ts

@@ -0,0 +1,13 @@
+import { Client } from '@elastic/elasticsearch';
+import { env } from './env.js';
+
+export const esClient = new Client({ node: env.ES_URL });
+
+export async function checkEsConnection(): Promise<void> {
+  try {
+    await esClient.ping();
+    console.log('Elasticsearch connected');
+  } catch (err) {
+    console.error('Elasticsearch connection failed:', err);
+  }
+}

services/msg/src/config/env.ts

@@ -0,0 +1,27 @@
+import { z } from 'zod';
+
+const envSchema = z.object({
+  PORT: z.string().default('3007'),
+  DATABASE_URL: z.string().url(),
+  REDIS_URL: z.string().url().optional(),
+  JWT_SECRET: z.string(),
+  JWT_ISSUER: z.string().default('next-edu-cloud'),
+  KAFKA_BROKERS: z.string().default('localhost:9092'),
+  ES_URL: z.string().url().default('http://localhost:9200'),
+  OTEL_EXPORTER_OTLP_ENDPOINT: z.string().url().optional(),
+  LOG_LEVEL: z.enum(['fatal', 'error', 'warn', 'info', 'debug', 'trace']).default('info'),
+  NODE_ENV: z.enum(['development', 'production', 'test']).default('development'),
+});
+
+export type Env = z.infer<typeof envSchema>;
+
+export function loadEnv(): Env {
+  const result = envSchema.safeParse(process.env);
+  if (!result.success) {
+    console.error('❌ Invalid environment variables:', result.error.flatten().fieldErrors);
+    throw new Error('Invalid environment configuration');
+  }
+  return result.data;
+}
+
+export const env = loadEnv();