重构: 迁移后端代码到 app 目录，前端移动到 WebUI，添加完整测试套件

主要变更:
- 后端代码从根目录迁移到 app/ 目录
- 前端代码从 frontend/ 重命名为 WebUI/
- 更新所有导入路径以适配新结构
- 提取公共 API 响应函数到 app/api/common.py
- 精简验证器服务代码
- 更新启动脚本和文档

测试:
- 新增完整测试套件 (tests/)
- 单元测试: 模型、仓库层
- 集成测试: 覆盖所有 22+ API 端点
- E2E 测试: 4个完整工作流场景
- 添加 pytest 配置和测试运行脚本

tests/integration/test_health_api.py

@@ -0,0 +1,47 @@
+"""健康检查 API 测试"""
+import pytest
+
+
+class TestHealthAPI:
+    """测试健康检查端点"""
+
+    @pytest.mark.asyncio
+    async def test_root_endpoint(self, client):
+        """测试根端点 /"""
+        response = await client.get("/")
+        assert response.status_code == 200
+        data = response.json()
+        assert "message" in data
+        assert "status" in data
+        assert data["status"] == "running"
+
+    @pytest.mark.asyncio
+    async def test_health_endpoint(self, client):
+        """测试健康检查端点 /health"""
+        response = await client.get("/health")
+        assert response.status_code == 200
+        data = response.json()
+        assert data["status"] == "healthy"
+        assert "timestamp" in data
+        assert "database" in data
+        assert "scheduler" in data
+        assert "version" in data
+
+    @pytest.mark.asyncio
+    async def test_health_endpoint_structure(self, client):
+        """测试健康检查端点返回结构"""
+        response = await client.get("/health")
+        data = response.json()
+        
+        assert "status" in data
+        assert "timestamp" in data
+        assert "database" in data
+        assert "scheduler" in data
+        assert "version" in data
+        
+        # 验证数据类型
+        assert isinstance(data["status"], str)
+        assert isinstance(data["timestamp"], str)
+        assert isinstance(data["database"], str)
+        assert isinstance(data["scheduler"], str)
+        assert isinstance(data["version"], str)