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/main.ts

@@ -0,0 +1,24 @@
+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);
+});