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

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+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 };
+  }
+}

services/msg/src/notifications/notifications.module.ts

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+import { Module } from '@nestjs/common';
+import { NotificationsController } from './notifications.controller.js';
+import { NotificationsService } from './notifications.service.js';
+
+@Module({
+  controllers: [NotificationsController],
+  providers: [NotificationsService],
+})
+export class NotificationsModule {}

services/msg/src/notifications/notifications.schema.ts

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+import { mysqlTable, varchar, char, timestamp, text, boolean, json } from 'drizzle-orm/mysql-core';
+
+export const notifications = mysqlTable('msg_notifications', {
+  id: char('id', { length: 36 }).notNull().primaryKey(),
+  userId: char('user_id', { length: 36 }).notNull(),
+  type: varchar('type', { length: 50 }).notNull(),
+  title: varchar('title', { length: 200 }).notNull(),
+  content: text('content').notNull(),
+  channel: varchar('channel', { length: 20 }).notNull().default('in_app'),
+  isRead: boolean('is_read').notNull().default(false),
+  metadata: json('metadata'),
+  createdAt: timestamp('created_at').notNull().defaultNow(),
+});
+
+export const notificationPreferences = mysqlTable('msg_notification_preferences', {
+  userId: char('user_id', { length: 36 }).notNull().primaryKey(),
+  emailEnabled: boolean('email_enabled').notNull().default(true),
+  smsEnabled: boolean('sms_enabled').notNull().default(false),
+  pushEnabled: boolean('push_enabled').notNull().default(true),
+  inAppEnabled: boolean('in_app_enabled').notNull().default(true),
+});
+
+export type Notification = typeof notifications.$inferSelect;
+export type NotificationPreference = typeof notificationPreferences.$inferSelect;

services/msg/src/notifications/notifications.service.ts

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+import { Injectable } from '@nestjs/common';
+import { getDb } from '../config/database.js';
+import { notifications, notificationPreferences } from './notifications.schema.js';
+import type { NotificationPreference } from './notifications.schema.js';
+import { v4 as uuidv4 } from 'uuid';
+import { eq, and } from 'drizzle-orm';
+import { esClient } from '../config/elasticsearch.js';
+import { logger } from '../shared/observability/logger.js';
+
+export interface SendNotificationDto {
+  userId: string;
+  type: string;
+  title: string;
+  content: string;
+  channel?: string;
+  metadata?: Record<string, unknown>;
+}
+
+@Injectable()
+export class NotificationsService {
+  async send(dto: SendNotificationDto) {
+    const id = uuidv4();
+    const db = getDb();
+
+    // 检查用户偏好
+    const [pref] = await db.select().from(notificationPreferences).where(eq(notificationPreferences.userId, dto.userId));
+
+    const channel = dto.channel || 'in_app';
+    if (pref && !this.isChannelEnabled(pref, channel)) {
+      logger.info({ userId: dto.userId, channel }, 'Notification skipped by preference');
+      return { skipped: true };
+    }
+
+    // 写入 DB
+    await db.insert(notifications).values({
+      id,
+      userId: dto.userId,
+      type: dto.type,
+      title: dto.title,
+      content: dto.content,
+      channel,
+      metadata: dto.metadata,
+    });
+
+    // 索引到 ES（全文检索）
+    try {
+      await esClient.index({
+        index: 'notifications',
+        id,
+        document: {
+          userId: dto.userId,
+          type: dto.type,
+          title: dto.title,
+          content: dto.content,
+          channel,
+          createdAt: new Date().toISOString(),
+        },
+      });
+    } catch (err) {
+      logger.error({ err }, 'Failed to index notification in ES');
+    }
+
+    // TODO: 触发 Push Gateway 推送（P5 后期）
+
+    return { id, skipped: false };
+  }
+
+  async listByUser(userId: string, onlyUnread: boolean = false) {
+    const db = getDb();
+    const conditions = onlyUnread
+      ? and(eq(notifications.userId, userId), eq(notifications.isRead, false))
+      : eq(notifications.userId, userId);
+    return db.select().from(notifications).where(conditions);
+  }
+
+  async markAsRead(id: string) {
+    const db = getDb();
+    await db.update(notifications).set({ isRead: true }).where(eq(notifications.id, id));
+  }
+
+  async search(userId: string, query: string) {
+    const result = await esClient.search({
+      index: 'notifications',
+      query: {
+        bool: {
+          must: [
+            { term: { userId } },
+            {
+              multi_match: {
+                query,
+                fields: ['title', 'content'],
+              },
+            },
+          ],
+        },
+      },
+    });
+    return result.hits.hits;
+  }
+
+  private isChannelEnabled(pref: NotificationPreference, channel: string): boolean {
+    switch (channel) {
+      case 'email': return pref.emailEnabled;
+      case 'sms': return pref.smsEnabled;
+      case 'push': return pref.pushEnabled;
+      case 'in_app': return pref.inAppEnabled;
+      default: return true;
+    }
+  }
+}