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/Configuration/ZugferdSettings.cs

@@ -0,0 +1,29 @@
+namespace DocumentOperator.Application.Common.Configuration;
+
+/// <summary>
+/// Configuration settings for ZUGFeRD/Factur-X/XRechnung detection
+/// </summary>
+public class ZugferdSettings
+{
+    /// <summary>
+    /// Exact ZUGFeRD/Factur-X/XRechnung file names to check (case-insensitive)
+    /// </summary>
+    public List<string> ZugferdFileNames { get; set; } = new()
+    {
+        "factur-x.xml",
+        "zugferd-invoice.xml",
+        "ZUGFeRD-invoice.xml",
+        "xrechnung.xml"
+    };
+    
+    /// <summary>
+    /// Partial filename patterns for ZUGFeRD detection (case-insensitive)
+    /// </summary>
+    public List<string> ZugferdFileNamePatterns { get; set; } = new()
+    {
+        "factur",
+        "zugferd",
+        "xrechnung",
+        "peppol"
+    };
+}