Improve file listing and fix notification metadata field

Backend:
- Optimize file listing for non-superusers with dedicated CRUD methods
- Add get_visible_for_user and count_visible_for_user for efficient queries
- Move /allowed-types/ and /max-size/ routes before /{file_id} for proper matching
- Rename notification 'metadata' field to 'extra_data' for clarity
- Fix settings export to use get_value() method

Frontend:
- Update NotificationItem interface to use extra_data field

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/api/v1/export.py

@@ -154,7 +154,7 @@ def export_settings_json(
         "exported_by": current_user.username,
         "count": len(settings_list),
         "settings": {
-            setting.key: setting.value
+            setting.key: setting.get_value()
             for setting in settings_list
         }
     }