全面架构重构：建立分层架构与高度可扩展的插件系统

后端重构：
- 新增分层架构：API Routes -> Services -> Repositories -> Infrastructure
- 彻底移除全局单例，全面采用 FastAPI 依赖注入
- 新增 api/ 目录拆分路由（proxies, plugins, scheduler, settings, stats）
- 新增 services/ 业务逻辑层：ProxyService, PluginService, SchedulerService, ValidatorService, SettingsService
- 新增 repositories/ 数据访问层：ProxyRepository, SettingsRepository, PluginSettingsRepository
- 新增 models/ 层：Pydantic Schemas + Domain Models
- 重写 core/config.py：采用 Pydantic Settings 管理配置
- 新增 core/db.py：基于 asynccontextmanager 的连接管理，支持数据库迁移
- 新增 core/exceptions.py：统一业务异常体系

插件系统重构（核心）：
- 新增 core/plugin_system/：BaseCrawlerPlugin + PluginRegistry
- 采用显式注册模式（装饰器 + plugins/__init__.py），类型安全、测试友好
- 新增 plugins/base.py：BaseHTTPPlugin 通用 HTTP 爬虫基类
- 迁移全部 7 个插件到新架构（fate0, proxylist_download, ip3366, ip89, kuaidaili, speedx, yundaili）
- 插件状态持久化到 plugin_settings 表

任务调度重构：
- 新增 core/tasks/queue.py：ValidationQueue + WorkerPool
- 解耦爬取与验证：爬虫只负责爬取，代理提交队列后由 Worker 异步验证
- 调度器定时从数据库拉取存量代理并分批投入验证队列

前端调整：
- 新增 frontend/src/services/ 层拆分 API 调用逻辑
- 调整 stores/ 和 views/ 使用 Service 层
- 保持 API 兼容性，页面无需大幅修改

其他：
- 新增 main.py 作为新入口
- 新增 DESIGN.md 架构设计文档
- 更新 requirements.txt 增加 pydantic-settings

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 core.db import get_db
+from repositories.proxy_repo import ProxyRepository
+from models.domain import Proxy
+from 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()