AO3-7506 Add backfill task for missing username pseuds

[pmonfort]

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Issue

https://otwarchive.atlassian.net/browse/AO3-7506

Purpose

Adds an After:backfill_missing_pseuds rake task that finds users who don't have a pseud matching their current username and creates one.

This covers the ~100 users who deleted their username-matching pseud before the fix in AO3-6359 (PR #5534) was deployed.

One difference from the SQL in the ticket: I added AND pseuds.user_id = users.id to the subquery. Without it, if another user happens to have a pseud with the same name as the affected user's login, the query would miss that user.

Testing Instructions

Testing instructions are in the Jira issue.

References

• AO3-6359 / AO3-6359 Fix Possible to delete the pseud that corresponds with your username if the capitalization or diacritics differ #5534 — the fix that prevents deleting the username-matching pseud

Credit

Pablo Monfort (he/him)

[sarken]

Drive-by question! What happens if the user login is Name and they have

• the pseud name (but not Name)
• the pseud Näme (but not Name)

Should we be testing those scenarios, since they're the ones that would've led the user to be able to delete the default pseud prior to PR #5534?