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

app/core/plugin_system/registry.py

@@ -0,0 +1,77 @@
+"""插件注册中心 - 显式注册，类型安全，测试友好"""
+import importlib
+import inspect
+import os
+from typing import Dict, List, Type, Optional
+from app.core.plugin_system.base import BaseCrawlerPlugin
+from app.core.log import logger
+
+
+class PluginRegistry:
+    """插件注册中心"""
+
+    def __init__(self):
+        self._plugins: Dict[str, Type[BaseCrawlerPlugin]] = {}
+        self._instances: Dict[str, BaseCrawlerPlugin] = {}
+
+    def register(self, plugin_cls: Type[BaseCrawlerPlugin]) -> Type[BaseCrawlerPlugin]:
+        """注册一个插件类。支持装饰器语法。"""
+        if not inspect.isclass(plugin_cls) or not issubclass(plugin_cls, BaseCrawlerPlugin):
+            raise ValueError("Plugin must be a subclass of BaseCrawlerPlugin")
+        if not plugin_cls.name:
+            raise ValueError(f"Plugin {plugin_cls.__name__} must have a 'name' attribute")
+        
+        self._plugins[plugin_cls.name] = plugin_cls
+        logger.info(f"Plugin registered: {plugin_cls.name} ({plugin_cls.__name__})")
+        return plugin_cls
+
+    def get(self, name: str) -> Optional[BaseCrawlerPlugin]:
+        """获取插件实例（懒加载）"""
+        if name not in self._instances:
+            cls = self._plugins.get(name)
+            if cls:
+                self._instances[name] = cls()
+        return self._instances.get(name)
+
+    def list_plugins(self) -> List[BaseCrawlerPlugin]:
+        """获取所有已注册插件的实例列表"""
+        result = []
+        for name in self._plugins:
+            instance = self.get(name)
+            if instance:
+                result.append(instance)
+        return result
+
+    def get_plugin_names(self) -> List[str]:
+        return list(self._plugins.keys())
+
+    def auto_discover(self, package_name: str):
+        """自动扫描指定包下的所有模块并注册其中的插件类。
+        注意：为了类型安全和可控性，推荐显式注册。auto_discover 仅作为兼容。"""
+        try:
+            package = importlib.import_module(package_name)
+            package_dir = os.path.dirname(package.__file__)
+        except Exception as e:
+            logger.error(f"Auto discover failed for package {package_name}: {e}")
+            return
+
+        for filename in os.listdir(package_dir):
+            if filename.endswith(".py") and not filename.startswith("__"):
+                module_name = f"{package_name}.{filename[:-3]}"
+                try:
+                    module = importlib.import_module(module_name)
+                    for attr_name in dir(module):
+                        obj = getattr(module, attr_name)
+                        if (
+                            inspect.isclass(obj)
+                            and issubclass(obj, BaseCrawlerPlugin)
+                            and obj is not BaseCrawlerPlugin
+                            and obj not in self._plugins.values()
+                        ):
+                            self.register(obj)
+                except Exception as e:
+                    logger.error(f"Failed to load module {module_name}: {e}")
+
+
+# 全局注册中心实例
+registry = PluginRegistry()