refactor(infra): S3 client singleton, ffmpeg pipe ordering, tidy streams/jobs

- Register IAmazonS3 as a singleton instead of scoped: the AWS SDK client is
  thread-safe and manages its own connection pool, so one instance per DI scope
  (per request / per job iteration) was wasteful and risked socket exhaustion
- AudioTranscoderService: start reading stdout/stderr before writing stdin to
  remove the latent pipe-buffer deadlock shape
- Dispose the resized-picture MemoryStream in ResizePictureActivity
- Background jobs use PeriodicTimer instead of while + Task.Delay

Verified: Worker boots against RabbitMQ + Postgres in Docker with no captive-
dependency error (dev scope validation) and both jobs start.

Build: 0 warnings, 0 errors.

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

backend/Musify.Infrastructure/MassTransit/Activities/Pictures/ResizePictureActivity.cs

@@ -30,7 +30,7 @@ namespace Musify.Infrastructure.MassTransit.Activities.Pictures
 
                 await using var fileStream = File.OpenRead(sourceFilePath);
 
-                var resizedPicture = await pictureHandler.ResizePictureAsWebpAsync(
+                await using var resizedPicture = await pictureHandler.ResizePictureAsWebpAsync(
                     fileStream,
                     executeContext.Arguments.Width,
                     executeContext.Arguments.Height,