fix: guard validate_unique against unsaved Release in admin add view

[JacobCoffee]

Summary

• Django 5.2 (deployed Feb 6 via deps: update to django 5.2 #2741) raises ValueError("Model instances passed to related filters must be saved.") when unsaved model instances are used in queryset filters
• ReleaseFile.validate_unique() filters by release=self.release, which fails on the admin add view because the parent Release hasn't been saved yet (no PK)
• Adding and self.release_id skips the uniqueness query for unsaved releases — safe because no conflicting records can exist for a release not yet in the database

Context

Steve Dower hit this today trying to create the "Python install manager 26.0" release via /admin/downloads/release/add/ with an inline MSIX release file that had download_button=True. Got 500 errors 3 times.

Sentry event: d05ec42eb5a44cceb69616c057186471

The validate_unique was added in #2137 (Sept 2022) and worked fine until the Django 5.2 upgrade tightened related filter validation.

Workaround (until this is deployed): create the release without release files or with download_button unchecked, save, then edit to add them.

Test plan

• Create a new release via /admin/downloads/release/add/ with an inline release file that has download_button=True — should save successfully
• Edit an existing release and toggle download_button — uniqueness validation should still fire correctly

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This pull request fixes a bug introduced by Django 5.2 where creating a Release with inline ReleaseFiles in the admin interface would fail with a ValueError. Django 5.2 (deployed Feb 6 via #2741) added stricter validation that prevents unsaved model instances from being used in queryset filters. The ReleaseFile.validate_unique() method was filtering by release=self.release, which failed when the parent Release hadn't been saved yet during admin add operations.

Changes:

• Added a guard condition to check self.release_id before running the uniqueness query in ReleaseFile.validate_unique()
• This prevents the ValueError while maintaining validation for saved releases

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