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

frontend/src/views/Settings.vue

@@ -190,7 +190,8 @@ import {
   VideoPause, 
   Refresh 
 } from '@element-plus/icons-vue'
-import { settingsAPI, schedulerAPI } from '../api'
+import { settingService } from '../services/settingService'
+import { schedulerService } from '../services/schedulerService'
 import PageHeader from '../components/PageHeader.vue'
 
 // ==================== 状态 ====================
@@ -237,7 +238,7 @@ const formRules = {
 async function fetchSettings() {
   loading.value = true
   try {
-    const response = await settingsAPI.getSettings()
+    const response = await settingService.getSettings()
     if (response.code === 200) {
       Object.assign(settings, response.data)
     }
@@ -251,7 +252,7 @@ async function fetchSettings() {
 
 async function fetchSchedulerStatus() {
   try {
-    const response = await schedulerAPI.getStatus()
+    const response = await schedulerService.getStatus()
     if (response.code === 200) {
       schedulerRunning.value = response.data.running
     }
@@ -264,7 +265,7 @@ async function fetchSchedulerStatus() {
 async function handleStartScheduler() {
   schedulerLoading.value = true
   try {
-    const response = await schedulerAPI.start()
+    const response = await schedulerService.start()
     if (response.code === 200) {
       schedulerRunning.value = true
       ElMessage.success('自动验证已启动')
@@ -282,7 +283,7 @@ async function handleStartScheduler() {
 async function handleStopScheduler() {
   schedulerLoading.value = true
   try {
-    const response = await schedulerAPI.stop()
+    const response = await schedulerService.stop()
     if (response.code === 200) {
       schedulerRunning.value = false
       ElMessage.success('自动验证已停止')
@@ -310,7 +311,7 @@ async function handleValidateNow() {
     )
     
     validating.value = true
-    const response = await schedulerAPI.validateNow()
+    const response = await schedulerService.validateNow()
     if (response.code === 200) {
       ElMessage.success('全量验证已启动，请在日志中查看进度')
     } else {
@@ -333,7 +334,7 @@ async function handleSave() {
   
   saving.value = true
   try {
-    const response = await settingsAPI.saveSettings(settings)
+    const response = await settingService.saveSettings(settings)
     
     if (response.code === 200) {
       ElMessage.success('配置保存成功')