Fix upload failure: add default storage backend to STORAGES

Django 5.x requires a 'default' key in STORAGES for FileField uploads.
Only 'staticfiles' was defined, causing InvalidStorageError on upload.

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

config/settings.py

@@ -90,6 +90,9 @@ USE_TZ = True
 STATIC_URL = '/static/'
 STATIC_ROOT = BASE_DIR / 'staticfiles'
 STORAGES = {
+    'default': {
+        'BACKEND': 'django.core.files.storage.FileSystemStorage',
+    },
     'staticfiles': {
         'BACKEND': 'whitenoise.storage.CompressedManifestStaticFilesStorage',
     },