fix: address PR review issues for catalogue images

- Use PNG format instead of JPEG for transparency support
- Non-strict mode only resizes if image width > target width
- Replace CATALOGUE_BACKGROUND_IMAGES_COLOR with CATALOGUE_BACKGROUND_IMAGES_RGBA (tuple default (0,0,0,0))
- Move all inline imports to top of test file
- Add test_resize_non_strict_no_upscale test case
- Add comment explaining settings.DEBUG guard for media serving

tienda_ilusion/config/urls.py

@@ -35,5 +35,7 @@ urlpatterns = [
     path('api/users/', include('users.urls')),
 ]
 
+# Serve media files through Django only in development.
+# In production/staging, media is served by the web server (nginx).
 if settings.DEBUG:
     urlpatterns += static(settings.MEDIA_URL, document_root=settings.MEDIA_ROOT)