Nested CSS Import URLs Not Presigned in S3 Storage Backend

[BlueMagma2]

Bug: Nested CSS Import URLs Not Presigned in S3 Storage Backend

Environment

• Django version: 4.2.16
• django-storages version: 1.14.3
• Python version: 3.11
• Backend: S3Storage

Description

When using the S3Storage backend, nested CSS import URLs (like @import url('widgets.css')) are not presigned, causing 403 Forbidden errors when loading static files from the Django admin interface.

Reproducible Example

1. Use Django admin static files
2. Load a CSS file with a relative @import statement
3. Observe that the imported CSS file fails to load without a presigned URL

Additional Context

• Direct static file URLs work correctly
• Presigning works for other file types
• Issue specifically affects nested/imported CSS files