Get Inventory Costs Summary

Headline cost figures for both own and consigned stock — what the Stock card shows once the user flips it to "com custos": vehicle count, total cost, and the purchase/expense split.

Same assembly as /inventory/costs, so the card and the dialog it opens always agree. Cacheable (unlike /costs itself) because the response is a handful of numbers, not the row set — same class as /inventory/summary. The FE only calls it when a card enters costs mode, so the underlying queries stay off the default dashboard load.