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.Domain/Models/ValueObjects/AnnotationOrigin.cs

@@ -0,0 +1,18 @@
+namespace DocumentService.Domain.Models.ValueObjects;
+
+/// <summary>
+/// Coordinate origin point for PDF annotations.
+/// PDF default is BottomLeft, but users may prefer TopLeft for easier UI integration.
+/// </summary>
+public enum AnnotationOrigin
+{
+    /// <summary>
+    /// Bottom-left corner (PDF native coordinate system, default)
+    /// </summary>
+    BottomLeft,
+    
+    /// <summary>
+    /// Top-left corner (common in UI frameworks)
+    /// </summary>
+    TopLeft
+}