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.

components/payments/paymentModel.js

@@ -4,19 +4,7 @@
 const mongoose = require('mongoose');
 
 const { getPayableAmount, normalizeDiscount } = require('../../utils/paymentAmount');
-
-const transactionSchema = new mongoose.Schema({
-  amount: { type: Number, required: true },
-  method: {
-    type: String,
-    enum: ['online', 'card', 'cash'],
-    default: 'card'
-  },
-  receiptNumber: { type: String, trim: true },
-  notes: { type: String, trim: true, maxlength: 5000 },
-  recordedBy: { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
-  date: { type: Date, default: Date.now }
-}, { _id: true });
+require('./transactionModel');
 
 const paymentSchema = new mongoose.Schema({
   user: {
@@ -56,13 +44,19 @@ const paymentSchema = new mongoose.Schema({
     enum: ['pending', 'partial', 'paid', 'overdue'],
     default: 'pending'
   },
-  transactions: [transactionSchema],
   notes: { type: String, trim: true, maxlength: 5000 }
 }, {
-  timestamps: true
+  timestamps: true,
+  toJSON: { virtuals: true },
+  toObject: { virtuals: true }
+});
+
+paymentSchema.virtual('transactions', {
+  ref: 'Transaction',
+  localField: '_id',
+  foreignField: 'payment'
 });
 
-// Auto-update status based on paid amount vs payable (amount - discount)
 paymentSchema.pre('save', function (next) {
   this.discount = normalizeDiscount(this.discount, this.amount);
   const payable = getPayableAmount(this);