Parts Spend Summary Route

Company-wide parts spend, per supplier and per month -- Frederick: "show parts spend per equipment unit, per supplier, and per month." Per-equipment spend is on GET /api/equipment/{id} and /history instead (parts_spend_cost/parts_spend_breakdown, the SAME "on the equipment page for that unit" ask), because it is scoped to one machine already in view, not a company-wide listing.