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/Jobs/TemporalUploadsCleanupJob.cs

@@ -19,7 +19,9 @@ namespace Musify.Infrastructure.Jobs
                 uploadIntentConfiguration.TemporalCleanUpJobIntervalSeconds,
                 uploadIntentConfiguration.TemporalRootPrefix);
 
-            while (!stoppingToken.IsCancellationRequested)
+            using var timer = new PeriodicTimer(TimeSpan.FromSeconds(uploadIntentConfiguration.TemporalCleanUpJobIntervalSeconds));
+
+            do
             {
                 try
                 {
@@ -29,9 +31,8 @@ namespace Musify.Infrastructure.Jobs
                 {
                     logger.LogError(exception, "Error in TemporalUploadsCleanUpJob");
                 }
-
-                await Task.Delay(TimeSpan.FromSeconds(uploadIntentConfiguration.TemporalCleanUpJobIntervalSeconds), stoppingToken);
             }
+            while (await timer.WaitForNextTickAsync(stoppingToken));
         }
 
         async Task RunAsync(CancellationToken cancellationToken)

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)

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,

backend/Musify.Infrastructure/Services/AudioTranscoderService.cs

@@ -103,13 +103,16 @@ namespace Musify.Infrastructure.Services
 
             logger.LogDebug("[{Operation}] ffmpeg started (PID: {ProcessId}) in {WorkingDirectory}", operationName, process.Id, workingDirectory);
 
+            var outputTask = process.StandardOutput.ReadToEndAsync(cancellationToken);
+            var errorTask = process.StandardError.ReadToEndAsync(cancellationToken);
+
             if (inputStream.CanSeek)
                 inputStream.Position = 0;
 
             await inputStream.CopyToAsync(process.StandardInput.BaseStream, cancellationToken);
             process.StandardInput.Close();
 
-            return await WaitForProcessResultAsync(process, operationName, cancellationToken);
+            return await WaitForProcessResultAsync(process, outputTask, errorTask, operationName, cancellationToken);
         }
 
         async Task<FfmpegExecutionResult> ExecuteFfmpegAsync(
@@ -123,7 +126,10 @@ namespace Musify.Infrastructure.Services
 
             logger.LogDebug("[{Operation}] ffmpeg started (PID: {ProcessId}) in {WorkingDirectory}", operationName, process.Id, workingDirectory);
 
-            return await WaitForProcessResultAsync(process, operationName, cancellationToken);
+            var outputTask = process.StandardOutput.ReadToEndAsync(cancellationToken);
+            var errorTask = process.StandardError.ReadToEndAsync(cancellationToken);
+
+            return await WaitForProcessResultAsync(process, outputTask, errorTask, operationName, cancellationToken);
         }
 
         Process BuildProcess(string arguments, string workingDirectory, bool redirectStandardInput)
@@ -144,12 +150,11 @@ namespace Musify.Infrastructure.Services
 
         async Task<FfmpegExecutionResult> WaitForProcessResultAsync(
             Process process,
+            Task<string> outputTask,
+            Task<string> errorTask,
             string operationName,
             CancellationToken cancellationToken)
         {
-            var errorTask = process.StandardError.ReadToEndAsync(cancellationToken);
-            var outputTask = process.StandardOutput.ReadToEndAsync(cancellationToken);
-
             using var linkedCancellationTokens = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
             linkedCancellationTokens.CancelAfter(audioTranscoderConfiguration.TranscodingTimeout);
 

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