Delete project and organization objects asynchronously

[stsewd]

Closes #10040

[stsewd]

Maybe one hour is fine? or less...

[Copilot]

Pull Request Overview

This PR converts project and organization deletion from synchronous to asynchronous operations to avoid request timeouts when deleting large objects. The changes introduce a new Celery task and view mixin to queue deletions for background processing.

Key Changes:

• Created AsyncDeleteViewWithMessage mixin to queue deletions asynchronously
• Added delete_object Celery task with locking to handle async deletions
• Enhanced set_change_reason to support user tracking outside request context

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
readthedocs/core/tasks.py	Adds delete_object Celery task to asynchronously delete database objects with locking mechanism
readthedocs/core/mixins.py	Introduces AsyncDeleteViewWithMessage mixin that queues deletion tasks instead of directly deleting objects
readthedocs/core/history.py	Extends set_change_reason to accept optional user parameter for tracking deletions outside request context
readthedocs/projects/views/private.py	Updates ProjectDelete view to use async deletion with updated success message
readthedocs/organizations/views/private.py	Updates DeleteOrganization view to use async deletion with updated success message

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