feat(p2): identity layer with IAM service and Teacher BFF

P2 阶段交付物：
- services/iam: 完整身份认证服务（users/roles/permissions/refresh_tokens 6 表 schema）
  - register/login/refresh/getUserInfo 4 个核心 API
  - bcrypt 密码哈希 + JWT 双 Token（access + refresh）
  - 复用 classes 黄金模板（errors/observability/middleware 三件套）
- services/teacher-bff: 教师聚合 BFF
  - Promise.allSettled 并行聚合 IAM + classes 数据
  - /teacher/dashboard 单一聚合端点
- packages/shared-proto/proto/iam.proto: IamService 契约（Register/Login/RefreshToken/GetUserInfo）
- api-gateway: 新增 IamServiceURL/TeacherBffURL 配置 + /iam/* + /teacher/* 路由

services/teacher-bff/src/app.module.ts

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+import { Module } from '@nestjs/common';
+import { TeacherModule } from './teacher/teacher.module.js';
+
+@Module({
+  imports: [TeacherModule],
+})
+export class AppModule {}

services/teacher-bff/src/config/env.ts

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+import { z } from 'zod';
+
+const envSchema = z.object({
+  PORT: z.string().default('3003'),
+  IamServiceUrl: z.string().url().default('http://localhost:3002'),
+  ClassesServiceUrl: z.string().url().default('http://localhost:3001'),
+  LOG_LEVEL: z.string().default('info'),
+  NODE_ENV: z.string().default('development'),
+});
+
+export type Env = z.infer<typeof envSchema>;
+
+export function loadEnv(): Env {
+  const result = envSchema.safeParse({
+    ...process.env,
+    IamServiceUrl: process.env.IAM_SERVICE_URL || 'http://localhost:3002',
+    ClassesServiceUrl: process.env.CLASSES_SERVICE_URL || 'http://localhost:3001',
+  });
+  if (!result.success) {
+    throw new Error('Invalid env: ' + JSON.stringify(result.error.flatten()));
+  }
+  return result.data;
+}
+
+export const env = loadEnv();

services/teacher-bff/src/main.ts

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+import 'reflect-metadata';
+import { NestFactory } from '@nestjs/core';
+import { AppModule } from './app.module.js';
+import { env } from './config/env.js';
+
+async function bootstrap(): Promise<void> {
+  const app = await NestFactory.create(AppModule, {
+    logger: ['log', 'error', 'warn'],
+  });
+
+  app.enableShutdownHooks();
+
+  await app.listen(env.PORT);
+  console.log(`Teacher BFF started on port ${env.PORT}`);
+
+  process.on('SIGTERM', async () => {
+    await app.close();
+  });
+}
+
+bootstrap().catch((err: unknown) => {
+  console.error('Failed to start Teacher BFF', err);
+  process.exit(1);
+});

services/teacher-bff/src/teacher/teacher.controller.ts

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+import { Controller, Get, Req } from '@nestjs/common';
+import type { Request } from 'express';
+import { TeacherService } from './teacher.service.js';
+
+interface AuthenticatedRequest extends Request {
+  userId?: string;
+}
+
+@Controller('teacher')
+export class TeacherController {
+  constructor(private readonly service: TeacherService) {}
+
+  @Get('dashboard')
+  async dashboard(@Req() req: Request) {
+    const authReq = req as AuthenticatedRequest;
+    const data = await this.service.getDashboard(authReq.userId as string);
+    return { success: true, data };
+  }
+}

services/teacher-bff/src/teacher/teacher.module.ts

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+import { Module } from '@nestjs/common';
+import { TeacherController } from './teacher.controller.js';
+import { TeacherService } from './teacher.service.js';
+
+@Module({
+  controllers: [TeacherController],
+  providers: [TeacherService],
+})
+export class TeacherModule {}

services/teacher-bff/src/teacher/teacher.service.ts

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+import { Injectable } from '@nestjs/common';
+import { env } from '../config/env.js';
+
+@Injectable()
+export class TeacherService {
+  // 聚合 IAM + classes 服务的数据
+  async getDashboard(userId: string): Promise<unknown> {
+    const [iamRes, classesRes] = await Promise.allSettled([
+      fetch(`${env.IamServiceUrl}/iam/me`, {
+        headers: { 'x-user-id': userId },
+      }),
+      fetch(`${env.ClassesServiceUrl}/classes`, {
+        headers: { 'x-user-id': userId },
+      }),
+    ]);
+
+    return {
+      user: iamRes.status === 'fulfilled' ? await iamRes.value.json() : null,
+      classes: classesRes.status === 'fulfilled' ? await classesRes.value.json() : null,
+    };
+  }
+}