fix(infra): repair TransferFiles compensation + tidy MassTransit consistency

Fix (recoverability bug):
- TransferFilesToBucketActivity returned Completed() with no log, so its
  compensation could never delete the transferred files (orphaned objects on
  fault). IStorageService.TransferFilesAsync now returns the real uploaded
  keys; the activity logs them so Compensate can remove them. Also drops the
  Path.Combine key recomputation (produced backslash keys on Windows).

Consistency / DRY:
- Normalize all courier activities to internal + folder-based namespaces
  (several were public and in the flat .Activities namespace)
- Extract the duplicated exponential retry into UseStandardRetry
- Unify endpoint-name logic into a single EndpointHelper (remove the parallel
  MassTransitEndpointNames helper)

Build: 0 warnings, 0 errors.

backend/Musify.Infrastructure/MassTransit/Activities/Files/TransferFilesToBucketActivity.cs

@@ -4,9 +4,9 @@ using Musify.Application.Contracts;
 using Musify.Infrastructure.MassTransit.Arguments;
 using Musify.Infrastructure.MassTransit.Logs;
 
-namespace Musify.Infrastructure.MassTransit.Activities
+namespace Musify.Infrastructure.MassTransit.Activities.Files
 {
-    public class TransferFilesToBucketActivity(
+    internal class TransferFilesToBucketActivity(
         IStorageService storageHandler,
         ILogger<TransferFilesToBucketActivity> logger)
         : IActivity<TransferFilesToBucketArguments, TransferFilesToBucketLog>
@@ -22,16 +22,15 @@ namespace Musify.Infrastructure.MassTransit.Activities
 
             try
             {
-                var uploadedKeys = Directory.GetFiles(folderPath)
-                    .Select(file => Path.Combine(destinationKey, Path.GetFileName(file)))
-                    .ToArray();
-
-                await storageHandler.TransferFilesAsync(
+                var uploadedKeys = await storageHandler.TransferFilesAsync(
                     folderPath,
                     executeContext.Arguments.DestinationBucket,
                     destinationKey,
                     executeContext.CancellationToken);
-                return executeContext.Completed();
+
+                return executeContext.Completed(new TransferFilesToBucketLog(
+                    executeContext.Arguments.DestinationBucket,
+                    uploadedKeys.ToArray()));
             }
             catch (Exception exception)
             {
@@ -63,4 +62,4 @@ namespace Musify.Infrastructure.MassTransit.Activities
             }
         }
     }
-}
+}

backend/Musify.Infrastructure/MassTransit/Activities/Files/UploadFileToBucketActivity.cs

@@ -6,7 +6,7 @@ using Musify.Infrastructure.MassTransit.Logs;
 
 namespace Musify.Infrastructure.MassTransit.Activities.Files
 {
-    public class UploadFileToBucketActivity(IStorageService storageHandler,
+    internal class UploadFileToBucketActivity(IStorageService storageHandler,
         ILogger<UploadFileToBucketActivity> logger)
         : IActivity<UploadFileToBucketArguments, UploadFileToBucketLog>
     {