feat(massEmails): query for users approaching submission limits

[rgraber]

🗒️ Checklist

1. run linter locally
2. update all related docs (API, README, inline, etc.), if any
3. draft PR with a title <type>(<scope>)<!>: <title> TASK-1234
4. tag PR: at least frontend or backend unless it's global
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6. review thyself: read the diff and repro the preview as written
7. open PR & confirm that CI passes
8. request reviewers, if needed
9. delete this section before merging

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