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

repositories/settings_repo.py

@@ -0,0 +1,102 @@
+"""设置数据访问层"""
+import json
+import aiosqlite
+from typing import Optional, Dict, Any
+from core.log import logger
+
+
+DEFAULT_SETTINGS = {
+    "crawl_timeout": 30,
+    "validation_timeout": 10,
+    "max_retries": 3,
+    "default_concurrency": 50,
+    "min_proxy_score": 0,
+    "proxy_expiry_days": 7,
+    "auto_validate": True,
+    "validate_interval_minutes": 30,
+}
+
+
+class SettingsRepository:
+    """系统设置 Repository"""
+
+    @staticmethod
+    async def get_all(db: aiosqlite.Connection) -> Dict[str, Any]:
+        settings = DEFAULT_SETTINGS.copy()
+        try:
+            async with db.execute("SELECT key, value FROM settings") as cursor:
+                rows = await cursor.fetchall()
+                for key, value in rows:
+                    # 类型转换
+                    default = DEFAULT_SETTINGS.get(key)
+                    if isinstance(default, bool):
+                        settings[key] = value.lower() == "true"
+                    elif isinstance(default, int):
+                        settings[key] = int(value)
+                    else:
+                        settings[key] = value
+        except Exception as e:
+            logger.error(f"get_all settings failed: {e}")
+        return settings
+
+    @staticmethod
+    async def save(db: aiosqlite.Connection, settings: Dict[str, Any]) -> bool:
+        try:
+            for key, value in settings.items():
+                await db.execute(
+                    """
+                    INSERT INTO settings (key, value, updated_at)
+                    VALUES (?, ?, CURRENT_TIMESTAMP)
+                    ON CONFLICT(key) DO UPDATE SET
+                        value = excluded.value,
+                        updated_at = CURRENT_TIMESTAMP
+                    """,
+                    (key, str(value)),
+                )
+            await db.commit()
+            return True
+        except Exception as e:
+            logger.error(f"save settings failed: {e}")
+            return False
+
+
+class PluginSettingsRepository:
+    """插件设置 Repository"""
+
+    @staticmethod
+    async def get_enabled(db: aiosqlite.Connection, plugin_id: str) -> Optional[bool]:
+        async with db.execute(
+            "SELECT enabled FROM plugin_settings WHERE plugin_id = ?", (plugin_id,)
+        ) as cursor:
+            row = await cursor.fetchone()
+            if row:
+                return bool(row[0])
+            return None
+
+    @staticmethod
+    async def set_enabled(db: aiosqlite.Connection, plugin_id: str, enabled: bool) -> bool:
+        try:
+            await db.execute(
+                """
+                INSERT INTO plugin_settings (plugin_id, enabled, created_at, updated_at)
+                VALUES (?, ?, CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)
+                ON CONFLICT(plugin_id) DO UPDATE SET
+                    enabled = excluded.enabled,
+                    updated_at = CURRENT_TIMESTAMP
+                """,
+                (plugin_id, int(enabled)),
+            )
+            await db.commit()
+            return True
+        except Exception as e:
+            logger.error(f"set_enabled failed for {plugin_id}: {e}")
+            return False
+
+    @staticmethod
+    async def list_all(db: aiosqlite.Connection) -> Dict[str, bool]:
+        result = {}
+        async with db.execute("SELECT plugin_id, enabled FROM plugin_settings") as cursor:
+            rows = await cursor.fetchall()
+            for plugin_id, enabled in rows:
+                result[plugin_id] = bool(enabled)
+        return result