refactor: Make Domain exceptions serializable and add XML docs

- Add [Serializable] attribute to all custom exceptions
- Add protected constructors for serialization support
- Add XML documentation comments
- Update SwissQrCodeNotFoundException message format

DocumentOperator.Domain/Common/Exceptions/DomainException.cs

@@ -4,6 +4,7 @@
 /// Base exception for all domain-related exceptions.
 /// Caught by the Exception Handling Middleware in the API layer.
 /// </summary>
+[Obsolete("This exception is deprecated. Use more specific exceptions for domain errors.")]
 public abstract class DomainException : Exception
 {
     /// <summary>