[WIP] Qf-6822 add restricted files update timestamp

[Rakanhf]

This change will make it easier for QField to detect if the project file has been changed on the cloud and whether it needs to be re-downloaded on the mobile.

---

A new field admin_restricted_data_last_updated_at is introduced, which will track date time whenever a restricted file (.qgs, .qgz, *_attachments.zip, .qgd, .qml) is modified.

[duke-nyuki]

Task linked: QF-6822 Add a new field Project.restricted_projectfiles_last_updated_at

[nirvn]

Super nice

[suricactus]

Please add a test before considering merge.

[suricactus]

@Rakanhf can you review if this is true and brind it to the attention to the dev group:

• I think when we delete a file, we do not update the data_last_updated_at, which is probably a mistake.

[suricactus]

This is in a mergeable state, don't take my last comment as a blocker.