refactor: reorganizar estructura del proyecto al estándar app/api_config

- core/ → app/api_config/
- apps/backend_admin/ → app/backend_admin/
- apps/common/ → app/common/
- apps/promociones/ → app/promociones/
- manage.py → app/manage.py
- Añadir app/requirements.txt
- Actualizar todos los imports y referencias (DJANGO_SETTINGS_MODULE, ROOT_URLCONF, WSGI_APPLICATION, INSTALLED_APPS)
- Actualizar Dockerfile con nuevo WORKDIR

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/api_config/urls.py

@@ -0,0 +1,8 @@
+from django.urls import path, include
+from backend_admin import views as admin_views
+
+urlpatterns = [
+    path('admin/', include('backend_admin.urls')),
+    path('promociones/', include('promociones.urls')),
+    path('api/token/', admin_views.api_token, name='token_obtain_pair'),
+]