7a12cba4d5
TaskStatus enum was missing 'cancelled' — the LLM tried to use it and
hit TaskStatus("cancelled") raising ValueError → 500. Added the value,
a migration to extend the task_status Postgres enum, and proper 400
validation guards on both create and update task routes.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>