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/Jobs/UploadIntentExpirationJob.cs

@@ -18,7 +18,9 @@ namespace Musify.Infrastructure.Jobs
         {
             logger.LogInformation("UploadIntentExpirationJob started (interval: {Interval}s)", uploadIntentConfiguration.ExpirationJobIntervalSeconds);
 
-            while (!stoppingToken.IsCancellationRequested)
+            using var timer = new PeriodicTimer(TimeSpan.FromSeconds(uploadIntentConfiguration.ExpirationJobIntervalSeconds));
+
+            do
             {
                 try
                 {
@@ -28,9 +30,8 @@ namespace Musify.Infrastructure.Jobs
                 {
                     logger.LogError(exception, "Error in UploadIntentExpirationJob");
                 }
-
-                await Task.Delay(TimeSpan.FromSeconds(uploadIntentConfiguration.ExpirationJobIntervalSeconds), stoppingToken);
             }
+            while (await timer.WaitForNextTickAsync(stoppingToken));
         }
 
         async Task RunAsync(CancellationToken cancellationToken)