[Feature] Merge frontend and backend uploads

[sky96111]

The feature

As mentioned in issue #7119 , frontend upload and backend upload are two different services.
Which raises the following issues when uploading huge files:

1. Users can't switch application to background during frontend upload (frontend uploading will be canceled)
2. Users can't open application during backend upload (frontend uploading will replace backend uploading, upload process will be reset)

Other apps such as Google Photos can switch apps into the backend and uploading still work.
A possible way is merge frontend and backend uploads into backend uploads, making frontend uploads only a trigger to start backend uploads immediately.

A greater idea is using WorkManager API, While it may be difficult in flutter-only app.
https://pub.dev/packages/workmanager maybe useful but I am not an export in Android dev and have no idea

Platform

• Server
• Web
• Mobile