Handle github repository transfer events.

[blkt]

Summary

Github webhook events having type repository and action transferred are handled by producing an internal event that triggers an evaluation. Despite being safe, it cannot lead to any useful action since the github app installation lost permissions to operate on the repository.

This change makes it so that repository.transferred events are handled as deletions. In order for such deletion to be successful, 403 Forbidden errors on webhook deletions must be handled gracefully, so that repository deletion can terminate without errors.

This is safe since (b) there's nothing Minder can do about this, having lost the necessary permissions, and (b) repository registration removes stale webhooks. The latter point is relevant to fix repository transfers when auto registration is enabled: in these cases, repository ownership is migrated before the event is delivered to Minder, making it impossible to cleanup webhooks, which leads to an SQL deletion failure, which eventually makes it impossibile to re-register the repository under the destination project.

Fixes #3274

Change Type

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Unit and manually tested.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 53.972% (+0.02%) from 53.952%
when pulling 5a6cfd4 on issue-3274
into ffddb6a on main.