Improve validations of users/attendee/availabilty to support required fields

Editor assessment

The subject states the validation improvement concretely: user, attendee, and availability validations now support required fields, which enumerates the schemas and the capability. The 'availabilty' typo is a small lapse that does not block comprehension. feat is right, and the missing scope is tolerable given the enumeration. No body or footer; the pull number gives provenance. Wording is long but grammatical. Honest and proportionate.