Add ZUGFeRD detection and extraction functionality

Introduced `ZugferdController` to handle ZUGFeRD-related operations, including detection and extraction of ZUGFeRD XML from PDFs via file upload or Base64-encoded payloads. Integrated `MediatR` for query/command handling.

Added `ZugferdSettings` for configurable file names and patterns, and updated `appsettings.json` and `Program.cs` to support this configuration.

Implemented `HasZugferdQuery` and `ExtractZugferdCommand` with their respective handlers and validators. Added DTOs (`ZugferdCheckResult`, `ZugferdExtractionResult`) for operation results.

Included `ZUGFeRD-Example.pdf` for testing and integrated `Serilog.Ui.Core.Extensions` for logging.

DocumentOperator.Application/Common/DTOs/ZugferdCheckResult.cs

@@ -0,0 +1,27 @@
+namespace DocumentOperator.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; }
+}