[FEATURE]: Allow re-ordering (sub)items in checklists via API

[Mittlebenskrise]

Describe the feature you'd like to see

In the UI you can re-order items in a checklist via drag and drop. It would be nice to also be able to change the order via the API. Since there already is an index number, a simple API call would be just setting this index for an item. The other items would then be shifted accordingly.

Wireframes or mockups (optional)

No response

Checklist

• I have searched across other issues to make sure this has not been already suggested.
• I understand this request may be declined if it is not in line with the project's goals.

[fccview]

Oh yeah, that should totally be doable, I'll sort it out ♥️