Add MediatR pipeline behaviors for SignCommand

Introduced MediatR to the project by adding the `using MediatR;` directive in `DependencyInjection.cs`. Registered pipeline behaviors for the `Signature.Commands.SignCommand` to enforce a structured execution order:

1. AnnotationBehavior: Saves annotations.
2. DocStatusBehavior: Creates document status.
3. HistoryBehavior: Records history.
4. SendSignedMailBehavior: Sends notification email (executes last).

These changes improve the command handling pipeline by ensuring sequential and reliable execution of behaviors.

EnvelopeGenerator.Application/DependencyInjection.cs

@@ -7,6 +7,7 @@ using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.DependencyInjection.Extensions;
 using QRCoder;
 using System.Reflection;
+using MediatR;
 
 namespace EnvelopeGenerator.Application;
 
@@ -56,6 +57,19 @@ public static class DependencyInjection
         services.AddMediatR(cfg =>
         {
             cfg.RegisterServicesFromAssembly(Assembly.GetExecutingAssembly());
+            
+            // Register SignCommand pipeline behaviors in execution order
+            // 1. AnnotationBehavior - Saves annotations (executes first)
+            cfg.AddBehavior<IPipelineBehavior<Signature.Commands.SignCommand, Unit>, Signature.Behaviors.AnnotationBehavior>();
+            
+            // 2. DocStatusBehavior - Creates document status (executes second)
+            cfg.AddBehavior<IPipelineBehavior<Signature.Commands.SignCommand, Unit>, Signature.Behaviors.DocStatusBehavior>();
+            
+            // 3. HistoryBehavior - Records history (executes third)
+            cfg.AddBehavior<IPipelineBehavior<Signature.Commands.SignCommand, Unit>, Signature.Behaviors.HistoryBehavior>();
+            
+            // 4. SendSignedMailBehavior - Sends notification email (executes LAST, only if all previous succeed)
+            cfg.AddBehavior<IPipelineBehavior<Signature.Commands.SignCommand, Unit>, Signature.Behaviors.SendSignedMailBehavior>();
         });
 
         return services;