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/iam/src/shared/observability/logger.ts

@@ -0,0 +1,19 @@
+import pino from 'pino';
+import { env } from '../../config/env.js';
+
+export const logger = pino({
+  level: env.LOG_LEVEL,
+  base: {
+    service: 'iam',
+    version: '0.1.0',
+  },
+  transport:
+    env.NODE_ENV === 'development'
+      ? {
+          target: 'pino-pretty',
+          options: { colorize: true },
+        }
+      : undefined,
+});
+
+export type Logger = typeof logger;