[OTE-863] update username generation query (backport #2482)

[mergify]

Changelist

[Describe or list the changes made in this PR]

Test Plan

[Describe how this PR was tested (if applicable)]

Author/Reviewer Checklist

• If this PR has changes that result in a different app state given the same prior state and transaction list, manually add the state-breaking label.
• If the PR has breaking postgres changes to the indexer add the indexer-postgres-breaking label.
• If this PR isn't state-breaking but has changes that modify behavior in PrepareProposal or ProcessProposal, manually add the label proposal-breaking.
• If this PR is one of many that implement a specific feature, manually label them all feature:[feature-name].
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• Manually add any of the following labels: refactor, chore, bug.

Summary by CodeRabbit

• Bug Fixes
  • Improved the efficiency of querying subaccounts without usernames by optimizing the SQL query structure.
• Chores
  • Maintained existing function signatures and public entity declarations for stability.

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This is an automatic backport of pull request #2482 done by [Mergify](https://mergify.com).

[linear]

OTE-863 Update username generation to be faster