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/api/deps.py

@@ -0,0 +1,45 @@
+"""依赖注入"""
+from fastapi import Request
+from app.services.proxy_service import ProxyService
+from app.services.plugin_service import PluginService
+from app.services.scheduler_service import SchedulerService
+from app.services.validator_service import ValidatorService
+from app.repositories.proxy_repo import ProxyRepository
+from app.core.tasks.queue import ValidationQueue
+from app.core.config import settings as app_settings
+
+
+def get_proxy_service() -> ProxyService:
+    return ProxyService()
+
+
+def get_plugin_service() -> PluginService:
+    return PluginService()
+
+
+def get_scheduler_service(request: Request) -> SchedulerService:
+    return request.app.state.scheduler_service
+
+
+def get_validation_queue(request: Request) -> ValidationQueue:
+    return request.app.state.validation_queue
+
+
+def create_scheduler_service() -> SchedulerService:
+    """在应用启动时创建 SchedulerService（非请求上下文）"""
+    validator = ValidatorService(
+        timeout=app_settings.validator_timeout,
+        connect_timeout=app_settings.validator_connect_timeout,
+        max_concurrency=app_settings.validator_max_concurrency,
+    )
+    proxy_repo = ProxyRepository()
+    queue = ValidationQueue(
+        validator=validator,
+        proxy_repo=proxy_repo,
+        worker_count=app_settings.validator_max_concurrency,
+        score_valid=app_settings.score_valid,
+        score_invalid=app_settings.score_invalid,
+        score_min=app_settings.score_min,
+        score_max=app_settings.score_max,
+    )
+    return SchedulerService(validation_queue=queue, proxy_repo=proxy_repo)