refactor: split models into modules, remove template-based views, and clean up code style

- Split monolithic models.py into models/ package (customers, products, sales, payments, admin)
- Removed forms.py, all HTML templates, and associated template-based views
- Added api/ package with CatalogSaleView placeholder
- Updated all imports across project to use new model paths
- Removed obsolete tests (form, export, purchase, summary tests)
- Removed template-based URL patterns, kept only API endpoints
- Standardized string quotes (single to double) and reformatted code

tienda_ilusion/don_confiao/api/sale_views.py

@@ -0,0 +1,5 @@
+from rest_framework import viewsets
+
+
+class CatalogSaleView(viewsets.ViewSet):
+    pass