Add admin transaction cancellation without deleting history.

Cancelled transactions are excluded from paid totals and due-date
calculations while remaining visible for audit purposes.

components/payments/transactionModel.js

@@ -35,7 +35,7 @@ const transactionSchema = new mongoose.Schema({
   },
   status: {
     type: String,
-    enum: ['pending', 'paid'],
+    enum: ['pending', 'paid', 'cancelled'],
     default: 'pending',
     index: true
   }