Create signup field values

Bulk-create or update signup field values for a student.

Each item in data identifies a signup field by id (obfuscated SignupField pk) and sets its value for the given student. Existing values are overwritten (upsert). Response is 200 if all items succeed, 207 if any fail, 400 if the batch is empty, 404 if the student is unknown or has no memberships in the organization.

Required OAuth scope: signup-fields:write