`ETag` is not updated when an attachment has been deleted from the files app

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Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior:

1. Add attachment xyz.jpg to any card
2. Synchronize with Deck Android app - you will see the image as a cover card and in the attachments section
3. Delete the xyz.jpg file from the Nextcloud Files Web UI or via the Nextcloud Files Android app
4. The attachment does no longer appear in the Deck Web UI
5. The attachment is still present on the Deck Android app because the ETag did not change and the Deck Android is not aware of the changed items
6. Temporarily disabling ETags in the Deck Android apps settings will make the attachment also disappear from the Deck Android app

Expected behavior
The Deck Web app should change the ETags when the file has been deleted just as it does when an attachment gets deleted from within the Deck Web app.

Screenshots

Deck: 1.8.3
Nextcloud version: 25.0.3

Context

Originally reported in the help forum: https://help.nextcloud.com/t/can-not-remove-cover-images-on-cards-by-deleting-imagefile-from-deck-folder/151670
Investigated within https://github.com/stefan-niedermann/nextcloud-deck/issues/1405

Reproduce the attachment deletion flow across the Deck Web UI, Nextcloud Files Web UI or Files Android app, and Deck Android app, focusing on ETag changes. Compare deleting an attachment in Deck with deleting its file externally; done means the ETag changes after external deletion so the Android client removes the attachment without disabling ETags.