Fix backfill script to handle zero-value costs and prices

[Copilot]

The backfill script used truthy checks that incorrectly skipped 0 values for purchase prices and maintenance costs.

Changes:

• Replace if (moto.purchasePrice) with if (moto.purchasePrice != null)
• Replace if (record.cost) with if (record.cost != null)

This ensures zero-cost items are normalized while still skipping null/undefined values.

// Before: skips valid zero values
if (moto.purchasePrice) {
    const normalizedPrice = moto.purchasePrice * factor;
    // ...
}

// After: only skips null/undefined
if (moto.purchasePrice != null) {
    const normalizedPrice = moto.purchasePrice * factor;
    // ...
}

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