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/admin.py

@@ -1,7 +1,9 @@
 from django.contrib import admin
-from .models import (
-    Customer, Sale, SaleLine, Product, ProductCategory, Payment,
-    ReconciliationJar)
+from .models.sales import Sale, SaleLine
+from .models.customers import Customer
+from .models.sales import Sale, SaleLine, Payment
+from .models.products import Product, ProductCategory
+from .models.payments import ReconciliationJar
 
 admin.site.register(Customer)
 admin.site.register(Sale)