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.test.js

@@ -6,7 +6,11 @@ const {
   getPayableAmount,
   normalizeDiscount,
   sanitizeNotes,
-  NOTES_MAX_LENGTH
+  NOTES_MAX_LENGTH,
+  rialsToToman,
+  isPaidTransaction,
+  sumPaidTransactions,
+  remainingPayable
 } = require('./paymentAmount');
 
 describe('payment amount helpers', () => {
@@ -36,6 +40,41 @@ describe('payment amount helpers', () => {
   });
 });
 
+describe('rialsToToman', () => {
+  it('drops one zero so spreadsheet Rials become website Toman', () => {
+    assert.equal(rialsToToman(110_000_000), 11_000_000);
+    assert.equal(rialsToToman(5_000_000), 500_000);
+    assert.equal(rialsToToman(0), 0);
+  });
+});
+
+describe('paid vs pending transactions', () => {
+  it('counts only paid transactions toward the paid total', () => {
+    const transactions = [
+      { amount: 8_000_000, status: 'paid', date: '2026-08-03' },
+      { amount: 3_000_000, status: 'pending', dueDate: '2026-08-23' }
+    ];
+    assert.equal(sumPaidTransactions(transactions), 8_000_000);
+    assert.equal(
+      remainingPayable({ amount: 11_000_000, discount: 0 }, transactions),
+      3_000_000
+    );
+  });
+
+  it('treats a 10m billed course as 1m off an 11m tuition', () => {
+    assert.equal(getPayableAmount({ amount: 11_000_000, discount: 1_000_000 }), 10_000_000);
+  });
+
+  it('treats an 8m billed course as 3m off an 11m tuition', () => {
+    assert.equal(getPayableAmount({ amount: 11_000_000, discount: 3_000_000 }), 8_000_000);
+  });
+
+  it('does not treat pending remainder rows as paid', () => {
+    assert.equal(isPaidTransaction({ status: 'pending', amount: 2_000_000 }), false);
+    assert.equal(isPaidTransaction({ status: 'paid', amount: 8_000_000 }), true);
+  });
+});
+
 describe('sanitizeNotes', () => {
   it('trims notes and caps length', () => {
     assert.equal(sanitizeNotes('  hello  '), 'hello');