Build Inventory into StayOnHire

### Discussed in https://github.com/orgs/stayonhire/discussions/74

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<sup>Originally posted by **dylanhru** November 11, 2023</sup>
Ben (Rustic hire):

"A quantity number on the assets - this would need to allow the confirmation of totals when adding to a job, which would then update an amount available for that period. This would be essential"
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