feat(config): ThemeResp 增加品牌字段 + 公开品牌信息端点

- ThemeResp 新增 brand_name/brand_slogan/brand_features/brand_copyright 字段
- default_theme 提供品牌默认值
- 新增 PublicBrandResp 和 GET /api/v1/public/brand 公开端点（无需认证）
- ConfigModule 增加 public_routes 方法
- 更新测试覆盖品牌字段

crates/erp-server/src/main.rs

@@ -535,13 +535,14 @@ async fn main() -> anyhow::Result<()> {
         ))
         .with_state(state.clone());
 
-    // Unthrottled public routes (health, docs) — no rate limiting
+    // Unthrottled public routes (health, docs, brand) — no rate limiting
     let unthrottled_routes = Router::new()
         .merge(handlers::health::health_check_router())
         .route(
             "/docs/openapi.json",
             axum::routing::get(handlers::openapi::openapi_spec),
         )
+        .merge(erp_config::ConfigModule::public_routes())
         .with_state(state.clone());
 
     // Clone jwt_secret for upload auth before protected_routes closure moves it