POST /tasks/{id}/move with section_id silently strips parent_id from subtasks

[KalebCole]

Description

When moving a subtask to a section via POST /api/v1/tasks/{task_id}/move with {"section_id": "..."}, the API silently clears the task's parent_id, promoting the subtask to a top-level task. The response returns 200 with no warning that the parent relationship was removed.

Steps to reproduce

1. Create a parent task and a subtask under it (parent_id set).
2. Call POST /api/v1/tasks/{subtask_id}/move with body {"section_id": "<section_id>"}.
3. Fetch the task again: parent_id is now null.

Expected behavior

Moving a subtask between sections should preserve its parent_id, or the API should return an error/warning if detaching the parent is intentional.

Actual behavior

The subtask is silently detached and becomes a top-level task in the target section.

Additional context

• Moving with {"parent_id": "<parent_id>"} on the same endpoint does re-attach the task, so the move endpoint clearly understands parentage. Only the section_id variant drops it.
• The task update endpoint (POST /api/v1/tasks/{task_id}) rejects parent_id as an unsupported field, so the move endpoint is currently the only way to restore the relationship after it is lost.
• Observed against REST API v1 on 2026-09-22.