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/plugins/proxylist_download.py

@@ -0,0 +1,55 @@
+from typing import List
+from app.core.plugin_system import ProxyRaw
+from app.plugins.base import BaseHTTPPlugin
+from app.core.log import logger
+
+
+class ProxyListDownloadPlugin(BaseHTTPPlugin):
+    name = "proxylist_download"
+    display_name = "ProxyListDownload"
+    description = "从 ProxyListDownload API 获取代理"
+
+    def __init__(self):
+        super().__init__()
+        self.urls = [
+            "https://www.proxy-list.download/api/v1/get?type=http",
+            "https://www.proxy-list.download/api/v1/get?type=https",
+            "https://www.proxy-list.download/api/v1/get?type=socks4",
+            "https://www.proxy-list.download/api/v1/get?type=socks5",
+        ]
+
+    async def crawl(self) -> List[ProxyRaw]:
+        results = []
+        for url in self.urls:
+            html = await self.fetch(url, timeout=30)
+            if not html:
+                continue
+
+            # 根据 URL 判断协议
+            if "type=socks4" in url:
+                protocol = "socks4"
+            elif "type=socks5" in url:
+                protocol = "socks5"
+            elif "type=https" in url:
+                protocol = "https"
+            else:
+                protocol = "http"
+
+            lines = html.split("\r\n")
+            if len(lines) <= 1:
+                lines = html.split("\n")
+
+            for line in lines:
+                line = line.strip()
+                if not line or ":" not in line:
+                    continue
+                parts = line.split(":")
+                if len(parts) >= 2:
+                    ip = parts[0].strip()
+                    port = parts[1].strip()
+                    if ip and port.isdigit():
+                        results.append(ProxyRaw(ip, int(port), protocol))
+
+        if results:
+            logger.info(f"{self.display_name} 解析完成，获得 {len(results)} 个潜在代理")
+        return results