refactor(backend): extract config and speedtest services (phase 4)

This commit continues the backend refactoring initiative by extracting
configuration management and API speedtest logic into dedicated service
layers, completing phase 4 of the architectural improvement plan.

## Changes

### New Service Layers
- **ConfigService** (`services/config.rs`): Consolidates all config
  import/export, backup management, and live sync operations
  - `create_backup()`: Creates timestamped backups with auto-cleanup
  - `export_config_to_path()`: Exports config to specified path
  - `load_config_for_import()`: Loads and validates imported config
  - `import_config_from_path()`: Full import with state update
  - `sync_current_providers_to_live()`: Syncs current providers to live files
  - Private helpers for Claude/Codex-specific sync logic

- **SpeedtestService** (`services/speedtest.rs`): Encapsulates endpoint
  latency testing with proper validation and error handling
  - `test_endpoints()`: Tests multiple URLs concurrently
  - URL validation now unified in service layer
  - Includes 3 unit tests for edge cases (empty list, invalid URLs, timeout clamping)

### Command Layer Refactoring
- Move all import/export commands to `commands/import_export.rs`
- Commands become thin wrappers: parse params → call service → return JSON
- Maintain `spawn_blocking` for I/O operations (phase 5 optimization)
- Lock acquisition happens after I/O completes (minimize contention)

### File Organization
- Delete: `import_export.rs`, `speedtest.rs` (root-level modules)
- Create: `commands/import_export.rs`, `services/config.rs`, `services/speedtest.rs`
- Update: Module declarations in `lib.rs`, `commands/mod.rs`, `services/mod.rs`

### Test Updates
- Update 20 integration tests in `import_export_sync.rs` to use `ConfigService` APIs
- All existing test cases pass without modification to test logic
- Add 3 new unit tests for `SpeedtestService`:
  - `sanitize_timeout_clamps_values`: Boundary value testing
  - `test_endpoints_handles_empty_list`: Empty input handling
  - `test_endpoints_reports_invalid_url`: Invalid URL error reporting

## Benefits

1. **Improved Testability**: Service methods are `pub fn`, easily callable
   from tests without Tauri runtime
2. **Better Separation of Concerns**: Business logic isolated from
   command/transport layer
3. **Enhanced Maintainability**: Related operations grouped in cohesive
   service structs
4. **Consistent Error Handling**: Services return `Result<T, AppError>`,
   commands convert to `Result<T, String>`
5. **Performance**: I/O operations run in `spawn_blocking`, locks released
   before file operations

## Testing

- ✅ All 43 tests passing (7 unit + 36 integration)
- ✅ `cargo fmt --check` passes
- ✅ `cargo clippy -- -D warnings` passes (zero warnings)

## Documentation

Updated `BACKEND_REFACTOR_PLAN.md` to reflect completion of config and
speedtest service extraction, marking phase 4 substantially complete.

Co-authored-by: Claude Code <code@anthropic.com>

docs/BACKEND_REFACTOR_PLAN.md

@@ -90,7 +90,9 @@
   - 新增 `services/provider.rs` 并实现 `ProviderService::switch` / `delete`，集中处理供应商切换、回填、MCP 同步等核心业务；命令层改为薄封装并在 `tests/provider_service.rs`、`tests/provider_commands.rs` 中完成成功与失败路径的集成验证。  
   - 新增 `services/mcp.rs` 提供 `McpService`，封装 MCP 服务器的查询、增删改、启用同步与导入流程；命令层改为参数解析 + 调用服务，`tests/mcp_commands.rs` 直接使用 `McpService` 验证成功与失败路径，阶段三测试继续适配。  
   - `McpService` 在内部先复制内存快照、释放写锁，再执行文件同步，避免阶段五升级后的 `RwLock` 在 I/O 场景被长时间占用；`upsert/delete/set_enabled/sync_enabled` 均已修正。  
-  - 仍待拆分的领域服务：配置导入导出、应用设置等命令需进一步抽象，以便统一封装文件 IO 与状态同步后再收尾阶段四。  
+  - 新增 `services/config.rs` 提供 `ConfigService`，统一处理配置导入导出、备份与 live 同步；命令层迁移至 `commands/import_export.rs`，在落盘操作前释放锁并复用现有集成测试。  
+  - 新增 `services/speedtest.rs` 并实现 `SpeedtestService::test_endpoints`，将 URL 校验、超时裁剪与网络请求封装在服务层，命令改为薄封装；补充单元测试覆盖空列表与非法 URL 分支。  
+  - 后续可选：应用设置（Store）命令仍较薄，可按需评估是否抽象；当前阶段四核心服务已基本齐备。  
 - **阶段 5：锁与阻塞优化 ✅（首轮）**  
   - `AppState` 已由 `Mutex<MultiAppConfig>` 切换为 `RwLock<MultiAppConfig>`，托盘、命令与测试均按读写语义区分 `read()` / `write()`；`cargo test` 全量通过验证并未破坏现有流程。  
   - 针对高开销 IO 的配置导入/导出命令提取 `load_config_for_import`，并通过 `tauri::async_runtime::spawn_blocking` 将文件读写与备份迁至阻塞线程，保持命令处理线程轻量。