feat(stamp): Add IPdfProcessor.AddStampAsync interface

- Add AddStampAsync method signature to IPdfProcessor interface
- Parameters: pdfBytes, stampType, pages, position, text/image/predefined params
- Supports Origin (TopLeft/BottomLeft), Rotation, Opacity, Placement

DocumentOperator.Application/Common/Interfaces/IPdfProcessor.cs

@@ -86,11 +86,12 @@ public interface IPdfProcessor
     /// </param>
     /// <param name="annotationType">Type of annotation to add (TextMarkup, FreeText, StickyNote, Circle, Square)</param>
     /// <param name="pageNumber">Page number (1-based) where annotation should be added</param>
-    /// <param name="rectangle">Annotation bounding rectangle (X1, Y1, X2, Y2) in PDF coordinates</param>
+    /// <param name="rectangle">Annotation bounding rectangle (X1, Y1, X2, Y2)</param>
     /// <param name="content">Annotation content/comment text (required for FreeText and StickyNote)</param>
     /// <param name="author">Optional author name</param>
     /// <param name="color">Optional annotation color in RGB format (hex string like "FF0000" for red)</param>
     /// <param name="textMarkupStyle">Text markup style (Highlight, Underline, Strikeout) - only for TextMarkup type</param>
+    /// <param name="origin">Coordinate origin (BottomLeft = PDF native, TopLeft = UI-friendly). Default: BottomLeft</param>
     /// <returns>Annotated PDF as byte array</returns>
     /// <exception cref="Domain.Common.Exceptions.BadRequestException">
     /// Thrown when stream is empty/invalid/not at Position=0, page number out of range, 
@@ -104,5 +105,44 @@ public interface IPdfProcessor
         string? content = null,
         string? author = null,
         string? color = null,
-        Domain.Models.ValueObjects.TextMarkupStyle? textMarkupStyle = null);
+        Domain.Models.ValueObjects.TextMarkupStyle? textMarkupStyle = null,
+        Domain.Models.ValueObjects.AnnotationOrigin origin = Domain.Models.ValueObjects.AnnotationOrigin.BottomLeft);
+
+    /// <summary>
+    /// Adds a stamp (text, image, or predefined) to specified pages of a PDF document.
+    /// </summary>
+    /// <param name="pdfStream">Input PDF stream (must support reading and seeking)</param>
+    /// <param name="stampType">Type of stamp (Text, Image, or Predefined)</param>
+    /// <param name="pageNumbers">Target page numbers (1-based). Null = all pages.</param>
+    /// <param name="position">Stamp position (X, Y coordinates)</param>
+    /// <param name="size">Stamp size (Width, Height). Null = auto-size for images.</param>
+    /// <param name="origin">Coordinate origin (BottomLeft or TopLeft)</param>
+    /// <param name="text">Text content (required for Text stamps)</param>
+    /// <param name="fontName">Font name (default: Arial)</param>
+    /// <param name="fontSize">Font size in points (default: 12)</param>
+    /// <param name="color">Hex color without # (e.g., "FF0000" for red, default: "000000")</param>
+    /// <param name="opacity">Opacity 0.0 (transparent) to 1.0 (opaque, default: 0.5)</param>
+    /// <param name="rotation">Rotation angle in degrees 0-360 (default: 0)</param>
+    /// <param name="placement">Foreground (on top) or Background (behind content)</param>
+    /// <param name="imageBytes">Image data (required for Image stamps, PNG/JPEG)</param>
+    /// <param name="predefinedType">Predefined stamp type (required for Predefined stamps)</param>
+    /// <returns>Stamped PDF as byte array</returns>
+    /// <exception cref="BadRequestException">Invalid parameters (missing text/image, invalid page numbers, invalid opacity/rotation)</exception>
+    /// <exception cref="PdfProcessingException">DevExpress processing error</exception>
+    Task<byte[]> AddStampAsync(
+        Stream pdfStream,
+        Domain.Models.ValueObjects.StampType stampType,
+        int[]? pageNumbers,
+        (double X, double Y) position,
+        (double Width, double Height)? size = null,
+        Domain.Models.ValueObjects.AnnotationOrigin origin = Domain.Models.ValueObjects.AnnotationOrigin.BottomLeft,
+        string? text = null,
+        string? fontName = null,
+        double? fontSize = null,
+        string? color = null,
+        double? opacity = null,
+        double? rotation = null,
+        Domain.Models.ValueObjects.StampPlacement placement = Domain.Models.ValueObjects.StampPlacement.Foreground,
+        byte[]? imageBytes = null,
+        Domain.Models.ValueObjects.PredefinedStampType? predefinedType = null);
 }