Forms: accept choice fields and return their options

The Forms module (#720, PR #821) was built before the choice field type (#803, PR #820) existed. rckoronadal.org's join form asks for an area of focus, and Color Me Fun asks for an entry type and shirt size, so a form must accept choice fields.

## Change

- `POST /api/public/forms/{slug}` accepts `choice` fields among the input types it allows, validated like any write: a value not offered is a 400 naming the accepted values.
- `GET /api/public/forms/{slug}` returns each choice field's `options` (value and label, in order) and `multiple`, so barakoPress renders a select, radios or checkboxes (BaryoDev/barakoPress#21).

## Done when

- A form with a single and a multiple choice field accepts offered values, refuses others with 400, and its GET lists the options in order.

## Where it lives

Module: `BarakoCMS.Forms`. It validates through the core's content validator, which already knows the choice type.

Start in the BarakoCMS.Forms module with the POST and GET handlers for /api/public/forms/{slug}, then trace their use of the core content validator for choice fields. Verify single and multiple choice inputs, including rejection of unoffered values with a 400, and confirm GET returns ordered option values and labels plus multiple.

Written by the indexing model from the issue text.