feat: implement plan upgrade logic with prorated pricing and MercadoPago integration

server/src/Models/PlanSubscriptions/PlanSubscriptions.pricing.ts

@@ -0,0 +1,11 @@
+const DEFAULT_BILLING_PERIOD_DAYS = 30;
+
+export function calculateProratedUpgradeAmount(
+    currentPlanPrice: number,
+    requestedPlanPrice: number,
+    remainingDays: number,
+    billingPeriodDays = DEFAULT_BILLING_PERIOD_DAYS
+): number {
+    const dailyDifference = (requestedPlanPrice - currentPlanPrice) / billingPeriodDays;
+    return Math.round(dailyDifference * remainingDays * 100) / 100;
+}