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/Configuration/ZugferdSettings.cs

@@ -0,0 +1,29 @@
+namespace DocumentService.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"
+    };
+}