isolated metadata keys constants

frontend/pweb/lib/providers/multiple_payouts.dart

@@ -15,6 +15,7 @@ import 'package:pweb/models/payment/multiple_payouts/csv_row.dart';
 import 'package:pweb/models/payment/multiple_payouts/state.dart';
 import 'package:pweb/utils/payment/multiple/csv_parser.dart';
 import 'package:pweb/utils/payment/multiple/intent_builder.dart';
+import 'package:pweb/utils/payment/upload_metadata.dart';
 
 
 class MultiplePayoutsProvider extends ChangeNotifier {
@@ -220,8 +221,10 @@ class MultiplePayoutsProvider extends ChangeNotifier {
         metadata: <String, String>{
           ...?_selectedFileName == null
               ? null
-              : <String, String>{'upload_filename': _selectedFileName!},
-          'upload_rows': _rows.length.toString(),
+              : <String, String>{
+                  UploadMetadataKeys.fileName: _selectedFileName!,
+                },
+          UploadMetadataKeys.rows: _rows.length.toString(),
           ...?_uploadAmountMetadata(),
         },
       );
@@ -276,8 +279,8 @@ class MultiplePayoutsProvider extends ChangeNotifier {
     final sentAmount = requestedSentAmount;
     if (sentAmount == null) return null;
     return <String, String>{
-      'upload_amount': sentAmount.amount,
-      'upload_currency': sentAmount.currency,
+      UploadMetadataKeys.amount: sentAmount.amount,
+      UploadMetadataKeys.currency: sentAmount.currency,
     };
   }
 
@@ -321,8 +324,8 @@ class MultiplePayoutsProvider extends ChangeNotifier {
     await quotation.quotePayments(
       intents,
       metadata: <String, String>{
-        'upload_filename': fileName,
-        'upload_rows': rows.length.toString(),
+        UploadMetadataKeys.fileName: fileName,
+        UploadMetadataKeys.rows: rows.length.toString(),
         ...?_uploadAmountMetadata(),
       },
     );