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/dataImport/dataImportController.js

@@ -18,8 +18,8 @@ exports.importJson = catchAsync(async (req, res) => {
   }
 
   // Allow { data: { courses: [...] } } wrappers
-  if (payload?.data?.courses) payload = payload.data;
-  if (!payload?.courses && payload?.version && payload?.courses == null) {
+  if (payload?.data?.courses || payload?.data?.payments) payload = payload.data;
+  if (!payload?.courses && !payload?.payments) {
     throw new AppError('VALIDATION_FAILED', null, 'Invalid import payload');
   }