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.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/Musify.Application/Contracts/IStorageService.cs

@@ -14,7 +14,7 @@ namespace Musify.Application.Contracts
 
         Task UploadFileAsync(Stream sourceStream, string contentType, string bucket, string key, CancellationToken cancellationToken);
 
-        Task TransferFilesAsync(string sourceDirectory, string bucket, string route, CancellationToken cancellationToken);
+        Task<IReadOnlyList<string>> TransferFilesAsync(string sourceDirectory, string bucket, string route, CancellationToken cancellationToken);
 
         Task RemoveFileAsync(string bucket, string key, CancellationToken cancellationToken);