refactor(backend): phase 5 - optimize concurrency with RwLock and async IO

Replace Mutex with RwLock for AppState.config to enable concurrent reads,
improving performance for tray menu building and query operations that
previously blocked each other unnecessarily.

Key changes:
- Migrate AppState.config from Mutex<MultiAppConfig> to RwLock<MultiAppConfig>
- Distinguish read-only operations (read()) from mutations (write()) across
  all command handlers and service layers
- Offload blocking file I/O in import/export commands to spawn_blocking threads,
  minimizing lock hold time and preventing main thread blocking
- Extract load_config_for_import() to separate I/O logic from state updates
- Update all integration tests to use RwLock semantics

Performance impact:
- Concurrent reads: Multiple threads can now query config simultaneously
  (tray menu, provider list, MCP config)
- Reduced contention: Write locks only acquired during actual mutations
- Non-blocking I/O: Config import/export no longer freezes UI thread

All existing tests pass with new locking semantics.

docs/BACKEND_REFACTOR_PLAN.md

@@ -89,6 +89,11 @@
 - **阶段 4：服务层抽象 🚧**  
   - 新增 `services/provider.rs` 并实现 `ProviderService::switch`，负责供应商切换时的业务流程（live 回填、持久化、MCP 同步），命令层通过薄封装调用并负责状态持久化。  
   - 扩展 `ProviderService` 提供 `delete` 能力，统一 Codex/Claude 清理逻辑；`tests/provider_service.rs` 校验切换与删除在成功/失败场景（包括缺失供应商、缺少 auth、删除当前供应商）下的行为，确保命令/托盘复用时拥有回归护栏。  
+- **阶段 5：锁与阻塞优化 🚧**  
+  - `AppState` 由 `Mutex<MultiAppConfig>` 切换为 `RwLock<MultiAppConfig>`，命令层根据读/写语义分别使用 `read()` 与 `write()`，避免查询场景被多余互斥阻塞。  
+  - 配套更新托盘初始化、服务层、MCP/Provider/Import Export 命令及所有集成测试，确保新锁语义下的并发安全；`cargo test` 全量通过（含命令、服务层集成用例）。  
+  - 针对可能耗时的配置导入/导出命令，抽取 `load_config_for_import` 负责文件 IO 与备份逻辑，并在命令层通过 `tauri::async_runtime::spawn_blocking` 下沉至阻塞线程执行，主线程仅负责状态写入与响应组装。  
+  - 其余命令（如设置查询、单文件读写）评估后维持同步执行，以免引入不必要的线程切换；后续若新增批量 IO 场景，再按同一模式挂载到阻塞线程。  
 
 ## 渐进式重构路线