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

plugins/base.py

@@ -0,0 +1,52 @@
+"""通用 HTTP 爬虫基类 - 为基于 HTTP 请求的插件提供封装"""
+import random
+import asyncio
+import aiohttp
+from typing import List
+from core.plugin_system import BaseCrawlerPlugin
+
+
+class BaseHTTPPlugin(BaseCrawlerPlugin):
+    """基于 HTTP 的爬虫插件基类"""
+
+    def __init__(self):
+        super().__init__()
+        self.user_agents = [
+            "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36",
+            "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/119.0.0.0 Safari/537.36",
+            "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/118.0.0.0 Safari/537.36",
+            "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/121.0",
+        ]
+        self.urls: List[str] = []
+        self.current_url: str = ""
+
+    def get_headers(self) -> dict:
+        return {
+            "User-Agent": random.choice(self.user_agents),
+            "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8",
+            "Accept-Language": "zh-CN,zh;q=0.8,zh-TW;q=0.7,zh-HK;q=0.5,en-US;q=0.3,en;q=0.2",
+            "Connection": "keep-alive",
+        }
+
+    async def fetch(self, url: str, timeout: float = 10.0, retries: int = 3) -> str:
+        """异步抓取指定 URL 的 HTML 内容"""
+        headers = self.get_headers()
+        async with aiohttp.ClientSession(headers=headers) as session:
+            for attempt in range(retries):
+                try:
+                    async with session.get(
+                        url, timeout=aiohttp.ClientTimeout(total=timeout)
+                    ) as response:
+                        if response.status == 200:
+                            content = await response.read()
+                            encoding = response.get_encoding()
+                            if encoding == "utf-8" or not encoding:
+                                try:
+                                    return content.decode("utf-8")
+                                except UnicodeDecodeError:
+                                    return content.decode("gbk", errors="ignore")
+                            return content.decode(encoding, errors="ignore")
+                except Exception:
+                    pass
+                await asyncio.sleep(random.uniform(1, 3))
+        return ""