Refactor: Remove background task management and related hooks

- Deleted `useBackgroundTasks.ts` hook and its associated logic for managing background tasks.
- Removed `backgroundTaskManager.ts` service and all related task definitions and registrations.
- Cleaned up `Podfile.lock` and `package.json` to remove unused dependencies related to background tasks.
- Updated iOS project files to eliminate references to removed background task components.
- Added new background fetch identifier in `Info.plist` for future use.

app/_layout.tsx

@@ -8,7 +8,6 @@ import 'react-native-reanimated';
 import PrivacyConsentModal from '@/components/PrivacyConsentModal';
 import { useAppDispatch, useAppSelector } from '@/hooks/redux';
 import { clearAiCoachSessionCache } from '@/services/aiCoachSession';
-import { backgroundTaskManager } from '@/services/backgroundTaskManager';
 import { notificationService } from '@/services/notifications';
 import { store } from '@/store';
 import { rehydrateUser, setPrivacyAgreed } from '@/store/userSlice';
@@ -31,16 +30,6 @@ function Bootstrapper({ children }: { children: React.ReactNode }) {
       await dispatch(rehydrateUser());
       setUserDataLoaded(true);
     };
-
-    const initializeBackgroundTasks = async () => {
-      try {
-        await backgroundTaskManager.initialize();
-        console.log('后台任务管理器初始化成功');
-      } catch (error) {
-        console.error('后台任务管理器初始化失败:', error);
-      }
-    };
-
     const initializeNotifications = async () => {
       try {
         // 初始化通知服务
@@ -52,7 +41,6 @@ function Bootstrapper({ children }: { children: React.ReactNode }) {
     };
 
     loadUserData();
-    initializeBackgroundTasks();
     initializeNotifications();
     // 冷启动时清空 AI 教练会话缓存
     clearAiCoachSessionCache();