feat(FinishEnvelopeJob): add PDF burning with concurrency control

- Introduced PDFBurnerParams configuration for concurrency limit.
- Added SemaphoreSlim to handle concurrent PDF burning for envelopes.
- Updated job to call _mediator.BurnPdf for each envelope with error handling.
- Preserved logging of job execution and envelope UUIDs.

EnvelopeGenerator.Application/Pdf/BurnPdfCommand.cs

@@ -32,7 +32,7 @@ public static class BurnPdfCommandExtensions
     /// <param name="envelopeId"></param>
     /// <param name="cancel"></param>
     /// <returns></returns>
-    public static Task<byte[]> BurnPdf(this ISender sender, int envelopeId, CancellationToken cancel) 
+    public static Task<byte[]> BurnPdf(this ISender sender, int envelopeId, CancellationToken cancel = default) 
         => sender.Send(new BurnPdfCommand(EnvelopeId: envelopeId), cancel);
 
     /// <summary>
@@ -42,7 +42,7 @@ public static class BurnPdfCommandExtensions
     /// <param name="envelopeUuid"></param>
     /// <param name="cancel"></param>
     /// <returns></returns>
-    public static Task<byte[]> BurnPdf(this ISender sender, string envelopeUuid, CancellationToken cancel)
+    public static Task<byte[]> BurnPdf(this ISender sender, string envelopeUuid, CancellationToken cancel = default)
         => sender.Send(new BurnPdfCommand(EnvelopeUuid: envelopeUuid), cancel);
 }