feat(domain): add domain exception hierarchy

- DomainException: Base exception for all domain errors
- ValidationException: Business rule validation failures
- AttachmentProcessingException: Attachment-specific processing errors

Exceptions maintain error code compatibility with legacy system.

src/DigitalData.EmailProfiler.Domain/Exceptions/DomainException.cs

@@ -0,0 +1,12 @@
+namespace DigitalData.EmailProfiler.Domain.Exceptions;
+
+public class DomainException : Exception
+{
+    public DomainException(string message) : base(message)
+    {
+    }
+
+    public DomainException(string message, Exception innerException) : base(message, innerException)
+    {
+    }
+}