🐛(backend) strip whitespace from media URLs in CORS proxy

When exporting a document to PDF, having whitespace before or after
the media URL causes the image to not be downloaded via the CORS proxy,
resulting in missing images in the exported PDF.
    
Signed-off-by: Mohamed El Amine BOUKERFA <boukerfa.ma@gmail.com>

src/backend/core/api/viewsets.py

@@ -2268,7 +2268,7 @@ class DocumentViewSet(
         GET /api/v1.0/documents/<resource_id>/cors-proxy
         Act like a proxy to fetch external resources and bypass CORS restrictions.
         """
-        url = request.query_params.get("url")
+        url = request.query_params.get("url", "").strip()
         if not url:
             return drf.response.Response(
                 {"detail": "Missing 'url' query parameter"},