feat(saas): P2 增强 — TOTP 2FA、Relay 重试、配置同步升级

- TOTP 2FA: totp-rs v5.7.1 + data-encoding Base32, setup/verify/disable 流程,
  登录时 TOTP 验证集成, SaasError::Totp 返回 400
- Relay 重试: 指数退避 (base_delay_ms * 2^attempt), 错误分类 (4xx 不重试),
  Admin POST /tasks/:id/retry 端点
- 配置同步: push (客户端覆盖) / merge (SaaS 优先) / diff (只读对比),
  实际写入 config_items 表
- 集成测试: 27 个测试全部通过 (新增 6 个 P2 测试)
- 文档: 更新 SaaS 平台总览 (模块完成度 + API 端点列表)

crates/zclaw-saas/src/error.rs

@@ -71,9 +71,10 @@ impl SaasError {
             Self::RateLimited(_) => StatusCode::TOO_MANY_REQUESTS,
             Self::Database(_) | Self::Internal(_) | Self::Io(_) | Self::Serialization(_) => StatusCode::INTERNAL_SERVER_ERROR,
             Self::AuthError(_) => StatusCode::UNAUTHORIZED,
-            Self::Jwt(_) | Self::PasswordHash(_) | Self::Totp(_) | Self::Encryption(_) => {
+            Self::Jwt(_) | Self::PasswordHash(_) | Self::Encryption(_) => {
                 StatusCode::INTERNAL_SERVER_ERROR
             }
+            Self::Totp(_) => StatusCode::BAD_REQUEST,
             Self::Config(_) => StatusCode::INTERNAL_SERVER_ERROR,
             Self::Relay(_) => StatusCode::BAD_GATEWAY,
         }