[Bug Report] persist sub file disappears always

### Checks

- [x] I have searched the existing issues
- [x] I have read the documentation
- [x] Is it your first time sumbitting an issue

### Current Behavior

I use persist file to save subscription for backup when network fails, with parameter:
persist_sub: 'https-file://www.example.com/persist_sub/link'

It runs well in most time.
However, sometimes the persist_sub disappears. Maybe after updating the subscription, reload or reboot.

The failure occurs many times, especially when the TIME YOU KNOW.

I didn't test why, but I think the condition is: my provider limits fetching of subscription link temporary. (but the relay/vpn links in previous subscription remains working. this is a popular policy in special time, you know).

### Expected Behavior

Whyever the reason is, I strongly recommand a fail-safe method to update the persist_sub. That is, fetch the subscription link and save it to a temp file first. If the fetching succeeds (after test the temp file size, file header, or something else), replace the old persis_file with the temp file.

Please kindly consider this proposal.

### Steps to Reproduce

service provider limits fetching subscription or network unstable.

### Environment

- **Dae version (use `dae --version`)**:
dae version v1.1.0
- **OS (e.g `cat /etc/os-release`)**:
Alpine Linux v3.22.3
- **Kernel (e.g. `uname -a`)**:
Linux alpiners 6.12.79-0-virt #1-Alpine SMP PREEMPT_DYNAMIC 2026-03-27 13:26:20 x86_64 Linux
- **Others**:

### Anything else?

_No response_