feat(saas): Phase 3 — 模型请求中转服务

- OpenAI 兼容 API 代理 (/api/v1/relay/chat/completions)
- 中转任务管理 (创建/查询/状态跟踪)
- 可用模型列表端点 (仅 enabled providers+models)
- 任务生命周期 (queued → processing → completed/failed)
- 用量自动记录 (token 统计 + 错误追踪)
- 3 个新集成测试覆盖中转端点

crates/zclaw-saas/src/main.rs

@@ -44,6 +44,7 @@ fn build_router(state: AppState) -> axum::Router {
     let protected_routes = zclaw_saas::auth::protected_routes()
         .merge(zclaw_saas::account::routes())
         .merge(zclaw_saas::model_config::routes())
+        .merge(zclaw_saas::relay::routes())
         .layer(middleware::from_fn_with_state(
             state.clone(),
             zclaw_saas::auth::auth_middleware,