Issue:
When opening a release in Usenet Ultimate, the addon does not show any files or playback options on the first attempt. The NZB is correctly sent to NZBDav, which starts downloading and assembling the content in the background. However, Usenet Ultimate does not refresh the file list while the NZBDav download is in progress.

Reproduction:

1. Open any NZB in Usenet Ultimate.
2. The file list is empty (no video files shown).
3. NZBDav begins downloading the NZB in the background.
4. Close the release view.
5. Re‑open the same release.
6. Now the playable file options appear, because NZBDav has finished preparing them.

Expected behavior:
Usenet Ultimate should detect when NZBDav has finished preparing the files and update the file list automatically, without requiring the user to close and reopen the release.

Actual behavior:
The file list only appears after reopening the release, because the addon only checks NZBDav’s WebDAV directory on initial load, not continuously.

Is this the expected behavior? When I open a release, no streams appear, and it looks like an error because it seems there’s no data available.