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/transactionModel.js

@@ -0,0 +1,48 @@
+// /components/payments/transactionModel.js
+'use strict';
+
+const mongoose = require('mongoose');
+
+const transactionSchema = new mongoose.Schema({
+  payment: {
+    type: mongoose.Schema.Types.ObjectId,
+    ref: 'Payment',
+    required: true,
+    index: true
+  },
+  user: {
+    type: mongoose.Schema.Types.ObjectId,
+    ref: 'User',
+    index: true
+  },
+  amount: {
+    type: Number,
+    required: true,
+    min: 0
+  },
+  method: {
+    type: String,
+    enum: ['online', 'card', 'cash']
+  },
+  receiptNumber: { type: String, trim: true },
+  notes: { type: String, trim: true, maxlength: 5000 },
+  recordedBy: { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
+  date: { type: Date },
+  dueDate: {
+    type: Date,
+    required: true,
+    index: true
+  },
+  status: {
+    type: String,
+    enum: ['pending', 'paid'],
+    default: 'pending',
+    index: true
+  }
+}, {
+  timestamps: true
+});
+
+transactionSchema.index({ payment: 1, dueDate: 1 });
+
+module.exports = mongoose.model('Transaction', transactionSchema);