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/config/env.ts

@@ -0,0 +1,25 @@
+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();