Fix supernav update on save

[JacobCoffee]

Description

• add postfave handler to that dont need to double save
• purge after release

[Copilot]

Pull request overview

This PR ensures the Downloads “supernav” and related download boxes refresh immediately when a ReleaseFile is added/changed/deleted, avoiding the need to “double save” a Release to see updates.

Changes:

• Add post_save and post_delete signal handlers for ReleaseFile to trigger supernav + box regeneration.
• Purge the Fastly /box/* endpoints when ReleaseFile changes on a published Release.
• Add model tests asserting box update functions are (or aren’t) triggered based on Release publish status.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
apps/downloads/models.py	Adds ReleaseFile save/delete signal handlers to refresh boxes and purge cached box URLs.
apps/downloads/tests/test_models.py	Adds tests verifying the new signal-driven updates run only for published releases.

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[Copilot]

The list of box purge URLs is duplicated here and in purge_fastly_download_pages() above. To avoid future drift (e.g., adding/removing a box in one place but not the other), consider centralizing these paths in a shared constant or a small helper used by both code paths.

[Copilot]

_update_boxes_for_release_file() dereferences instance.release to check is_published, which can introduce an extra DB query on each ReleaseFile save/delete when the Release isn’t already cached. If this handler ends up being called frequently, consider checking publication status via a lightweight query (e.g., Release.objects.filter(pk=instance.release_id, is_published=True).exists()) to avoid loading the full Release object.

Suggested change

	if instance.release_id and instance.release.is_published:
	if instance.release_id and Release.objects.filter(pk=instance.release_id, is_published=True).exists():