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.

backend/Musify.Infrastructure/Services/ServicesDependencyInjection.cs

@@ -21,7 +21,7 @@ namespace Musify.Infrastructure.Services
             serviceDescriptors.AddSingleton(serviceProvider =>
                 serviceProvider.GetRequiredService<IOptions<InfrastructureStorageConfiguration>>().Value);
 
-            serviceDescriptors.AddScoped<IAmazonS3>(serviceProvider =>
+            serviceDescriptors.AddSingleton<IAmazonS3>(serviceProvider =>
             {
                 var configuration = serviceProvider.GetRequiredService<InfrastructureStorageConfiguration>();
                 var s3Configuration = new AmazonS3Config