feat: store payment installments in a transactions collection

Move embedded payment rows into Transaction documents with due dates so remaining tuition can be tracked separately from paid amounts.

utils/paymentAmount.js

@@ -22,9 +22,34 @@ const sanitizeNotes = (notes) => {
   return String(notes).trim().slice(0, NOTES_MAX_LENGTH);
 };
 
+const rialsToToman = (value) => Math.floor(toNonNegativeNumber(value) / 10);
+
+const isPaidTransaction = (trx = {}) => {
+  const status = String(trx.status || '').toLowerCase();
+  if (status === 'pending') return false;
+  if (status === 'paid') return true;
+  return Boolean(trx.date);
+};
+
+const sumPaidTransactions = (transactions = []) => {
+  if (!Array.isArray(transactions)) return 0;
+  return transactions.reduce((sum, trx) => {
+    if (!isPaidTransaction(trx)) return sum;
+    return sum + toNonNegativeNumber(trx.amount);
+  }, 0);
+};
+
+const remainingPayable = (payment = {}, transactions = []) => {
+  return Math.max(0, getPayableAmount(payment) - sumPaidTransactions(transactions));
+};
+
 module.exports = {
   NOTES_MAX_LENGTH,
   getPayableAmount,
   normalizeDiscount,
-  sanitizeNotes
+  sanitizeNotes,
+  rialsToToman,
+  isPaidTransaction,
+  sumPaidTransactions,
+  remainingPayable
 };