Refactor item validation and calculation logic

- Moved validation functions for creating and updating line items to dedicated modules.
- Introduced new calculations module for handling financial calculations related to line items.
- Updated import paths for validation functions to reflect new structure.

backend/api/v1/modules/a76/invoices/services.py

@@ -126,7 +126,10 @@ class InvoiceService:
 
         # Validar si la factura ya existe
         invoice_exists(db, invoice_data.invoice_number, tenant_id, company_id, errors)
-        validate_create_import(db, invoice_data, tenant_id, company_id, errors)
+        if invoice_data.operation_type == "exp":
+            validate_create_export(db, invoice_data, tenant_id, company_id, errors)
+        else:
+            validate_create_import(db, invoice_data, tenant_id, company_id, errors)
 
         # Si hay errores, lanzar excepción ANTES de intentar crear
         errors.raise_if_errors("Error al crear la factura")