Preserve original usernames on historical messages after account deletion

Description: As an admin/compliance reviewer, I want historical messages to display the original username for posts authored by users who are later deleted, so past conversations remain intelligible and auditable. Problem: When a user is deleted, our UI replaces their name on past messages with a generated "deleted12345" label. This obscures who said what, hinders incident reviews, and creates confusion for teams reviewing historical activity. While Support can manually restore a specific user's name, this is not scalable. Benefit: Preserving the original username on historical messages maintains clear context and auditability across conversations. It reduces support load, speeds investigations, and improves user trust by keeping conversation history accurate without manual intervention.