fix: make user balance initialization idempotent (#19)

[suguanYang]

Summary

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• describe any API, worker, deployment, or migration impact
• link the related issue or task

Verification

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Deployment Notes

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Checklist

• Tests were added or updated when behavior changed
• Public docs, examples, or OpenAPI contracts were updated when needed
• Database migrations are idempotent and safe to deploy
• Logs, errors, and validation paths avoid leaking secrets or user data
• The pull request description explains any breaking or user-visible change