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/PredefinedStampType.cs

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+namespace DocumentOperator.Domain.Models.ValueObjects;
+
+/// <summary>
+/// Predefined stamp types with standard text and styling.
+/// </summary>
+public enum PredefinedStampType
+{
+    /// <summary>
+    /// CONFIDENTIAL stamp (red, bold).
+    /// </summary>
+    Confidential,
+
+    /// <summary>
+    /// APPROVED stamp (green, bold).
+    /// </summary>
+    Approved,
+
+    /// <summary>
+    /// DRAFT stamp (gray, italic).
+    /// </summary>
+    Draft,
+
+    /// <summary>
+    /// VOID stamp (red, strikethrough effect).
+    /// </summary>
+    Void,
+
+    /// <summary>
+    /// FOR REVIEW stamp (orange, bold).
+    /// </summary>
+    ForReview
+}

DocumentOperator.Domain/Models/ValueObjects/StampPlacement.cs

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+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
+}

DocumentOperator.Domain/Models/ValueObjects/StampType.cs

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+namespace DocumentOperator.Domain.Models.ValueObjects;
+
+/// <summary>
+/// Specifies the type of stamp to add to a PDF document.
+/// </summary>
+public enum StampType
+{
+    /// <summary>
+    /// Text-based stamp with custom text, font, and color.
+    /// </summary>
+    Text,
+
+    /// <summary>
+    /// Image-based stamp (PNG/JPEG overlay).
+    /// </summary>
+    Image,
+
+    /// <summary>
+    /// Predefined stamp with standard text (e.g., CONFIDENTIAL, APPROVED).
+    /// </summary>
+    Predefined
+}