Refactor: Rename DocumentOperator to DocumentService

Updated project and solution files to rename and restructure
`DocumentOperator` to `DocumentService`. Modified project
references in `DocumentService.API.csproj`, `DocumentService.Application.csproj`,
and `DocumentService.Infrastructure.csproj` to reflect the new
naming convention. Updated `DocumentService.sln` to remove
old project references, add new ones, and adjust configuration
and nested project mappings accordingly. This change aligns
the project structure with the new naming convention.

DocumentService.Application/DependencyInjection.cs

@@ -0,0 +1,44 @@
+using FluentValidation;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace DocumentService.Application;
+
+/// <summary>
+/// Dependency Injection configuration for Application Layer
+/// </summary>
+public static class DependencyInjection
+{
+    /// <summary>
+    /// Registers Application Layer services (MediatR, FluentValidation, AutoMapper, Behaviors)
+    /// </summary>
+    public static IServiceCollection AddApplication(this IServiceCollection services, IConfiguration configuration)
+    {
+        var assembly = typeof(DependencyInjection).Assembly;
+
+        // Read LuckyPennySoft license key from appsettings.json
+        var licenseKey = configuration.GetValue<string>("LuckyPennySoftLicenseKey")
+            ?? throw new InvalidOperationException("LuckyPennySoftLicenseKey not found in configuration");
+
+        // Register MediatR (scannt Assembly nach Handlers)
+        services.AddMediatR(config =>
+        {
+            config.LicenseKey = licenseKey;
+            config.RegisterServicesFromAssembly(assembly);
+
+            // Pipeline Behaviors (Reihenfolge wichtig!)
+            config.AddOpenBehavior(typeof(Common.Behaviors.ValidationBehavior<,>));
+            config.AddOpenBehavior(typeof(Common.Behaviors.LoggingBehavior<,>));
+        });
+
+        // Register FluentValidation (scannt Assembly nach Validators)
+        services.AddValidatorsFromAssembly(assembly);
+
+        // Register AutoMapper (scannt Assembly nach Profiles)
+        services.AddAutoMapper(cfg => {
+            cfg.LicenseKey = licenseKey;
+        }, typeof(Common.Mapping.MappingProfile));
+
+        return services;
+    }
+}