When creating an inbox, `usage_limits` eagerly computes captain limits
even though only the `:inboxes` value is needed. If `total_count` in
`get_captain_limits` resolves to a negative number, `clamp(0, negative)`
raises `ArgumentError: min argument must be less than or equal to max
argument`, causing a 500 on inbox creation.
Ensure `total_count` is floored at 0 so the clamp range is always valid.
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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