Fix 500 on settings update: include user_id in explicit SettingsResponse constructor

The W9 fix added user_id to SettingsResponse but missed the manual
_to_settings_response() builder, causing Pydantic validation failure.

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

backend/app/routers/settings.py

@@ -17,6 +17,7 @@ def _to_settings_response(s: Settings) -> SettingsResponse:
     """
     return SettingsResponse(
         id=s.id,
+        user_id=s.user_id,
         accent_color=s.accent_color,
         upcoming_days=s.upcoming_days,
         preferred_name=s.preferred_name,