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/paymentController.js

@@ -58,3 +58,9 @@ exports.updateTransaction = catchAsync(async (req, res, next) => {
   const payment = await paymentService.updateTransaction(req.params.transactionId, req.body, actorId);
   return successResponse(res, 200, 'Transaction updated successfully', payment);
 });
+
+exports.cancelTransaction = catchAsync(async (req, res, next) => {
+  const actorId = req.user?._id;
+  const payment = await paymentService.cancelTransaction(req.params.transactionId, actorId);
+  return successResponse(res, 200, 'Transaction cancelled successfully', payment);
+});