fix: mysql referential integrity error in update_user during retire_forum

[Alam-2U]

Description

After migrating the forum backend from MongoDB to MySQL, the retire_forum API started failing due to MySQL-specific referential integrity constraints that were not properly handled in the existing implementation.

A previous fix was introduced in PR #38, but the issue persists. The failure occurs because user.save() attempts to update the email field in the auth_user table, which has already been modified by LMS, leading to conflicts.

This PR addresses the issue by ensuring that updates to auth_user do not violate existing constraints and by aligning the deletion/update flow with MySQL’s referential integrity requirements.

Ticket

COSMO2-917

[Copilot]

Pull request overview

Fixes user retirement failures after the MongoDB → MySQL migration by preventing unintended updates to auth_user fields that can violate MySQL constraints during retire_forum.

Changes:

• Limit Django User.save() calls in the MySQL backend to only update username (via update_fields) and skip saving when no user fields changed.
• Adjust the retire-user flow to only blank email for non-MySQL backends (MongoDB), since LMS owns auth_user.email in the MySQL deployment.
• Bump package version to 0.5.2.

Reviewed changes

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

File	Description
forum/backends/mysql/api.py	Restricts User.save() to username updates (and only when needed) to avoid touching email and triggering constraint issues.
forum/api/users.py	Makes retirement email-blanking conditional by backend type (MongoDB vs MySQL).
forum/__init__.py	Version bump to reflect the fix release.

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