离线下载bt有多个文件要全部下载完才会转存

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### 1. Is this request related to a challenge you're experiencing? Tell me about your story.

能不能先把下载进度100%的文件转存了，再下载其他文件？

### 2. Additional context or comments

_No response_

### 3. Can you help us with this feature?

- [ ] I am interested in contributing to this feature.

No source files or tests are named. Start by tracing the backend flow for offline BT downloads and the subsequent file-transfer step, then identify how completion of individual files is handled. Done means files that reach 100% can be transferred before the remaining files finish, with regression coverage for multi-file BT downloads.

Written by the indexing model from the issue text.