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/teacher/teacher.service.ts

@@ -0,0 +1,22 @@
+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,
+    };
+  }
+}