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/Common/DTOs/ZugferdCheckResult.cs

@@ -0,0 +1,27 @@
+namespace DocumentService.Application.Common.DTOs;
+
+/// <summary>
+/// DTO for ZUGFeRD check result
+/// </summary>
+public record ZugferdCheckResult
+{
+    /// <summary>
+    /// Indicates whether the PDF contains ZUGFeRD XML attachment
+    /// </summary>
+    public bool HasZugferd { get; init; }
+
+    /// <summary>
+    /// ZUGFeRD XML file name (e.g., "factur-x.xml")
+    /// </summary>
+    public string? ZugferdFileName { get; init; }
+
+    /// <summary>
+    /// ZUGFeRD XML file size in bytes
+    /// </summary>
+    public long? ZugferdFileSize { get; init; }
+
+    /// <summary>
+    /// MIME type of the ZUGFeRD XML file
+    /// </summary>
+    public string? ZugferdMimeType { get; init; }
+}