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)

Cargo.lock

@@ -965,6 +965,12 @@ dependencies = [
  "parking_lot_core",
 ]
 
+[[package]]
+name = "data-encoding"
+version = "2.10.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d7a1e2f27636f116493b8b860f5546edb47c8d8f8ea73e1d2a20be88e28d1fea"
+
 [[package]]
 name = "der"
 version = "0.7.10"
@@ -7432,6 +7438,7 @@ dependencies = [
  "axum-extra",
  "chrono",
  "dashmap",
+ "data-encoding",
  "futures",
  "hex",
  "jsonwebtoken",
@@ -7453,6 +7460,7 @@ dependencies = [
  "tracing",
  "tracing-subscriber",
  "url",
+ "urlencoding",
  "uuid",
  "zclaw-types",
 ]