Implement deletion workflow action with soft/hard delete options and validation:
- Add WorkflowActionDeletion model with skip_trash flag
- Add DELETION action type to WorkflowAction enum
- Handle document deletion in workflow signal handlers
- Add validation to ensure deletion action is always last in workflow
- Skip remaining workflows if document was deleted
- Clean up orphaned WorkflowActionDeletion records
- Add comprehensive test coverage for deletion action validation and execution
