feat: fuel surcharge per lane and cargo type

apps/edr-freight-api/src/modules/rule-engine/rule-engine.service.ts

@@ -648,9 +648,12 @@ export class RuleEngineService {
   /**
    * Fuel surcharge — fires when the booking's cargo type has hasFuel = true,
    * billed off the FUEL rate matching the booking's lane (trade direction +
-   * origin + destination) and cargo type. PER_LITER bills baseLiters ×
-   * rateValue once per booking; PER_WAGON bills the wagons the cargo occupies.
-   * No matching lane rate simply bills nothing — same leniency as lashing.
+   * origin + destination) and cargo type. PER_LITER collapses to one FLAT
+   * amount (baseLiters × rateValue, once per booking) — the customer only ever
+   * sees the total, and the frozen contract snapshot stores that same flat
+   * figure so the snapshot-override math bills it exactly once. PER_WAGON
+   * bills the wagons the cargo occupies. No matching lane rate simply bills
+   * nothing — same leniency as lashing.
    */
   private fuelCharges(
     input: BookingEvaluationInput,
@@ -673,9 +676,12 @@ export class RuleEngineService {
       0,
       Number(input.bulkWagons ?? 0) || Number(input.totalWagons ?? 0),
     );
-    const billedQty =
-      rate.rateUnit === 'PER_LITER' ? Number(rate.baseLiters ?? 0) : wagons;
-    const amount = billedQty * rateValue;
+    const perLiter = rate.rateUnit === 'PER_LITER';
+    const billedQty = perLiter ? 1 : wagons;
+    const unitPrice = perLiter
+      ? Number(rate.baseLiters ?? 0) * rateValue
+      : rateValue;
+    const amount = billedQty * unitPrice;
     if (!(amount > 0)) return modifiers;
     modifiers.push({
       rateId: rate.id,
@@ -683,8 +689,8 @@ export class RuleEngineService {
       triggerValue: billedQty,
       calculatedAmount: amount,
       currency: rate.currency,
-      unitPriceUsd: rateValue,
-      billingUnit: rate.rateUnit,
+      unitPriceUsd: unitPrice,
+      billingUnit: perLiter ? 'FLAT' : rate.rateUnit,
     });
     return modifiers;
   }