refactor(domain): remove obsolete exception types

Deleted:
- PdfProcessingException: Obsolete, DevExpress exceptions now propagate naturally
- SwissQrCodeNotFoundException: Moved to Application layer (feature-specific exception)

Rationale:
- PdfProcessingException was wrapping library exceptions unnecessarily
- Better to let infrastructure exceptions propagate → middleware handles as 500
- SwissQrCodeNotFoundException is application-level concern, not domain

DocumentOperator.Domain/Common/Exceptions/PdfProcessingException.cs

@@ -1,35 +0,0 @@
-namespace DocumentOperator.Domain.Common.Exceptions;
-
-/// <summary>
-/// Exception thrown when PDF processing operations fail.
-/// Maps to HTTP 500 Internal Server Error or 422 Unprocessable Entity in the API layer.
-/// </summary>
-[Obsolete("This exception is deprecated. Use more specific exceptions for PDF processing errors.")]
-public class PdfProcessingException : DomainException
-{
-    public string Operation { get; }
-
-    public PdfProcessingException(string operation, string message)
-        : base($"PDF processing failed during '{operation}': {message}", "PDF_PROCESSING_ERROR")
-    {
-        Operation = operation;
-    }
-
-    public PdfProcessingException(string operation, string message, Exception innerException)
-        : base($"PDF processing failed during '{operation}': {message}", "PDF_PROCESSING_ERROR", innerException)
-    {
-        Operation = operation;
-    }
-
-    public PdfProcessingException(string message)
-        : base(message, "PDF_PROCESSING_ERROR")
-    {
-        Operation = "Unknown";
-    }
-
-    public PdfProcessingException(string message, Exception innerException)
-        : base(message, "PDF_PROCESSING_ERROR", innerException)
-    {
-        Operation = "Unknown";
-    }
-}

DocumentOperator.Domain/Exceptions/SwissQrCodeNotFoundException.cs

@@ -1,23 +0,0 @@
-namespace DocumentOperator.Domain.Exceptions;
-
-/// <summary>
-/// Exception thrown when a Swiss QR Code cannot be found in a PDF document.
-/// </summary>
-[Obsolete("This exception is deprecated. Use SwissQrCodeNotFoundException instead.")]
-public sealed class SwissQrCodeNotFoundException : Exception
-{
-    public SwissQrCodeNotFoundException()
-        : base("No Swiss QR Code found on the last page of the PDF document.")
-    {
-    }
-
-    public SwissQrCodeNotFoundException(string message)
-        : base(message)
-    {
-    }
-
-    public SwissQrCodeNotFoundException(string message, Exception innerException)
-        : base(message, innerException)
-    {
-    }
-}