Set STORAGE_URL so uploaded files get public URLs

Without this, files were saved with http://localhost:3000/uploads/... URLs
which are unreachable from the browser. Now uses the public backend domain.

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

docker-compose.yml

@@ -5,6 +5,7 @@ services:
       - '3001:3001'
     environment:
       - PORT=3001
+      - STORAGE_URL=https://backend.prosapp.co/uploads
 
   admin:
     build: