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/Pictures/UpdateTrackPictureActivity.cs

@@ -5,9 +5,9 @@ using Musify.Domain.ValueObjects;
 using Musify.Infrastructure.MassTransit.Arguments;
 using Musify.Infrastructure.MassTransit.Logs;
 
-namespace Musify.Infrastructure.MassTransit.Activities
+namespace Musify.Infrastructure.MassTransit.Activities.Pictures
 {
-    public class UpdateTrackPictureActivity(
+    internal class UpdateTrackPictureActivity(
         IDatabase database,
         ILogger<UpdateTrackPictureActivity> logger)
         : IActivity<UpdateTrackPictureArguments, UpdateTrackPictureLog>