feat(expenses): add beloPrice and usdcEquivalent fields to Expense model and update related logic

apps/backend/prisma/schema.prisma

@@ -130,6 +130,8 @@ model Expense {
   month             Int
   amount            Decimal          @db.Money
   amountPayed       Decimal?         @db.Money
+  beloPrice         Decimal?         @db.Money
+  usdcEquivalent    Decimal?         @db.Decimal(12, 6)
   status            PaymentStatus    @default(PENDING)
   dueDate           DateTime
   paymentDate       DateTime?