fix w kalkulacji last price

app/domain/costs.py

@@ -12,3 +12,25 @@ def calculate_costs(gross, vat_rate=23, vat_deduction_percent=50):
     deductible_vat = vat * deduction
     return {"gross": money(gross), "net": money(net), "vat": money(vat),
             "deductible_vat": money(deductible_vat), "final_cost": money(gross - deductible_vat)}
+
+
+def calculate_net_price_costs(net_price, vat_rate=23, vat_deduction_percent=50):
+    """Return per-unit invoice and effective costs for a net fuel price.
+
+    A 50% VAT deduction means the company bears half of the VAT, not a 50% VAT rate.
+    """
+    net = Decimal(str(net_price))
+    rate = Decimal(str(vat_rate)) / Decimal("100")
+    deduction = Decimal(str(vat_deduction_percent)) / Decimal("100")
+    deduction = min(max(deduction, Decimal("0")), Decimal("1"))
+    vat = net * rate
+    invoice_gross = net + vat
+    deductible_vat = vat * deduction
+    effective_cost = invoice_gross - deductible_vat
+    return {
+        "net": net,
+        "vat": vat,
+        "invoice_gross": invoice_gross,
+        "deductible_vat": deductible_vat,
+        "effective_cost": effective_cost,
+    }