feat(stamp): Add Domain value objects for stamp operations

- Add StampType enum (Text, Image, Predefined)
- Add PredefinedStampType enum (CONFIDENTIAL, APPROVED, DRAFT, etc.)
- Add StampPlacement enum (Foreground, Background)
- All enums in DocumentOperator.Domain namespace

DocumentOperator.Domain/Models/ValueObjects/StampPlacement.cs

@@ -0,0 +1,17 @@
+namespace DocumentOperator.Domain.Models.ValueObjects;
+
+/// <summary>
+/// Specifies whether the stamp should appear in the foreground (on top of content) or background (behind content).
+/// </summary>
+public enum StampPlacement
+{
+    /// <summary>
+    /// Stamp appears on top of existing page content.
+    /// </summary>
+    Foreground,
+
+    /// <summary>
+    /// Stamp appears behind existing page content (watermark effect).
+    /// </summary>
+    Background
+}