Backend:
- Add due_time (TIME, nullable) column to todos model + migration 015
- Add due_time to Create/Update/Response schemas
Frontend:
- Add due_time to Todo type
- TodoForm: add time input, convert empty strings to null before
sending (fixes date appearing required — Pydantic rejected '' as date)
- TodoItem: display clock icon + time when due_time is set
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
