refactor: 重构异步任务系统和计费服务架构

- 重构任务系统：新增 lifecycle (TaskStatus/BillingStatus)、context、application 模块
- 将 video tasks 泛化为 async tasks，支持更通用的异步任务管理
- 新增 Gemini Files 管理模块和管理界面
- 重构 billing 服务：拆分 schema.py 和 service.py
- 新增 candidate 服务模块用于请求候选管理
- 数据库迁移：添加 billing_status、request_id、gemini_file_mappings 表和索引
- 移除废弃的 video_telemetry、task orchestrator 等模块

frontend/src/router/index.ts

@@ -208,10 +208,20 @@ const routes: RouteRecordRaw[] = [
         name: 'AnnouncementManagement',
         component: () => importWithRetry(() => import('@/views/user/Announcements.vue'))
       },
+      {
+        path: 'async-tasks',
+        name: 'AsyncTasks',
+        component: () => importWithRetry(() => import('@/views/admin/AsyncTasks.vue'))
+      },
+      {
+        path: 'gemini-files',
+        name: 'GeminiFilesManagement',
+        component: () => importWithRetry(() => import('@/views/admin/GeminiFilesManagement.vue'))
+      },
+      // 保留旧路由兼容性
       {
         path: 'video-tasks',
-        name: 'VideoTasks',
-        component: () => importWithRetry(() => import('@/views/admin/VideoTasks.vue'))
+        redirect: '/admin/async-tasks'
       }
     ]
   }