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/services/proxy_service.py

@@ -0,0 +1,93 @@
+"""代理业务服务"""
+import csv
+import json
+import io
+from datetime import datetime
+from typing import List, Optional, Tuple, AsyncIterator
+from app.core.db import get_db
+from app.repositories.proxy_repo import ProxyRepository
+from app.models.domain import Proxy
+from app.core.log import logger
+
+
+class ProxyService:
+    def __init__(self, proxy_repo: ProxyRepository = ProxyRepository()):
+        self.proxy_repo = proxy_repo
+
+    async def get_stats(self) -> dict:
+        async with get_db() as db:
+            stats = await self.proxy_repo.get_stats(db)
+            stats["today_new"] = await self.proxy_repo.get_today_new_count(db)
+            return stats
+
+    async def list_proxies(
+        self,
+        page: int = 1,
+        page_size: int = 20,
+        protocol: Optional[str] = None,
+        min_score: int = 0,
+        max_score: Optional[int] = None,
+        sort_by: str = "last_check",
+        sort_order: str = "DESC",
+    ) -> Tuple[List[Proxy], int]:
+        async with get_db() as db:
+            return await self.proxy_repo.list_paginated(
+                db, page, page_size, protocol, min_score, max_score, sort_by, sort_order
+            )
+
+    async def get_random_proxy(self) -> Optional[Proxy]:
+        async with get_db() as db:
+            return await self.proxy_repo.get_random(db)
+
+    async def delete_proxy(self, ip: str, port: int) -> None:
+        async with get_db() as db:
+            await self.proxy_repo.delete(db, ip, port)
+
+    async def batch_delete(self, proxies: List[Tuple[str, int]]) -> int:
+        async with get_db() as db:
+            return await self.proxy_repo.batch_delete(db, proxies)
+
+    async def clean_invalid(self) -> int:
+        async with get_db() as db:
+            return await self.proxy_repo.clean_invalid(db)
+
+    async def clean_expired(self, days: int) -> int:
+        async with get_db() as db:
+            return await self.proxy_repo.clean_expired(db, days)
+
+    async def export_proxies(
+        self,
+        fmt: str,
+        protocol: Optional[str] = None,
+        limit: int = 10000,
+    ) -> AsyncIterator[str]:
+        async with get_db() as db:
+            proxies = await self.proxy_repo.list_all(db, protocol=protocol, limit=limit)
+
+        if fmt == "csv":
+            yield "IP,Port,Protocol,Score,Last Check\n"
+            for p in proxies:
+                yield f"{p.ip},{p.port},{p.protocol},{p.score},{self._fmt_time(p.last_check)}\n"
+        elif fmt == "txt":
+            for p in proxies:
+                yield f"{p.ip}:{p.port}\n"
+        elif fmt == "json":
+            data = [
+                {
+                    "ip": p.ip,
+                    "port": p.port,
+                    "protocol": p.protocol,
+                    "score": p.score,
+                    "last_check": self._fmt_time(p.last_check),
+                }
+                for p in proxies
+            ]
+            yield json.dumps(data, ensure_ascii=False, indent=2)
+
+    @staticmethod
+    def _fmt_time(dt: Optional[datetime]) -> str:
+        if not dt:
+            return ""
+        if isinstance(dt, str):
+            return dt
+        return dt.isoformat()