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/Common/Configurations/PDFBurnerParams.cs

@@ -9,6 +9,11 @@ namespace EnvelopeGenerator.Application.Common.Configurations;
 /// </summary>
 public class PDFBurnerParams : ITextStyle
 {
+    /// <summary>
+    /// 
+    /// </summary>
+    public int ConcurrencyLimit { get; set; } = 5;
+
     /// <summary>
     /// 
     /// </summary>