Add interfaces and DTOs for PDF operations support

Introduced multiple interfaces and DTOs to support a wide range of
PDF-related operations, including attachment handling, conversion,
validation, annotation, stamping, and metadata extraction.

Key changes:
- Added `DocumentServiceClientOptions` for HTTP client config.
- Introduced `IPdfAttachmentClient` for attachment operations.
- Added `IPdfConversionClient` for PDF/A conversion (marked obsolete).
- Added `IPdfOperationsClient` for merge, annotate, and stamp ops.
- Introduced `IPdfValidationClient` for PDF and PDF/A validation.
- Added `ISwissQrCodeClient` for Swiss QR Code extraction.
- Added `IZugferdClient` for ZUGFeRD detection and extraction.
- Created request DTOs for Base64-encoded operations.
- Added enums for annotation, stamp, and validation configurations.

These changes provide a flexible and extensible foundation for
interacting with PDF documents, supporting both multipart and
Base64-encoded inputs, and ensuring type safety with enums and records.

DocumentService.Client/Interfaces/IPdfOperationsClient.cs

@@ -0,0 +1,69 @@
+using DocumentService.Client.Models.Requests;
+
+namespace DocumentService.Client.Interfaces;
+
+/// <summary>
+/// Client for PDF operations (merge, annotate, stamp).
+/// </summary>
+public interface IPdfOperationsClient
+{
+    // ==================== MERGE OPERATIONS ====================
+    
+    /// <summary>
+    /// Merges multiple PDFs (multipart).
+    /// </summary>
+    /// <param name="pdfStreams">PDF file streams to merge</param>
+    /// <param name="pageRanges">Optional page ranges per PDF (e.g., "1-3,5")</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Merged PDF as stream</returns>
+    Task<Stream> MergeAsync(IEnumerable<Stream> pdfStreams, List<string?>? pageRanges = null, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Merges multiple PDFs (Base64).
+    /// </summary>
+    /// <param name="pdfByteArrays">PDF files as byte arrays</param>
+    /// <param name="pageRanges">Optional page ranges per PDF (e.g., "1-3,5")</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Merged PDF as stream</returns>
+    Task<Stream> MergeAsync(IEnumerable<byte[]> pdfByteArrays, List<string?>? pageRanges = null, CancellationToken cancellationToken = default);
+
+    // ==================== ANNOTATION OPERATIONS ====================
+    
+    /// <summary>
+    /// Adds annotation to PDF (multipart).
+    /// </summary>
+    /// <param name="pdfStream">PDF file stream</param>
+    /// <param name="request">Annotation request with coordinates and style</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Annotated PDF as stream</returns>
+    Task<Stream> AnnotateAsync(Stream pdfStream, AddAnnotationBase64Request request, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Adds annotation to PDF (Base64).
+    /// </summary>
+    /// <param name="pdfBytes">PDF file as byte array</param>
+    /// <param name="request">Annotation request with coordinates and style</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Annotated PDF as stream</returns>
+    Task<Stream> AnnotateAsync(byte[] pdfBytes, AddAnnotationBase64Request request, CancellationToken cancellationToken = default);
+
+    // ==================== STAMP OPERATIONS ====================
+    
+    /// <summary>
+    /// Adds stamp to PDF (multipart).
+    /// </summary>
+    /// <param name="pdfStream">PDF file stream</param>
+    /// <param name="request">Stamp request with position and style</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Stamped PDF as stream</returns>
+    Task<Stream> StampAsync(Stream pdfStream, AddStampBase64Request request, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Adds stamp to PDF (Base64).
+    /// </summary>
+    /// <param name="pdfBytes">PDF file as byte array</param>
+    /// <param name="request">Stamp request with position and style</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Stamped PDF as stream</returns>
+    Task<Stream> StampAsync(byte[] pdfBytes, AddStampBase64Request request, CancellationToken cancellationToken = default);
+}