As survey/program admin I want to mark a dimension as required

Requested by Ropecon 2026 in their post-con feedback.

Some dimensions should be markable as required, so that a subject (program item, survey response) cannot be accepted or saved in a workflow-complete state without a value in that dimension.

Background: during program offer acceptance, dimension values were sometimes left missing, and nothing flagged this. Related to dimension value defaults on the acceptance form (#1021) — defaults reduce the chance of missing values, requiredness catches the rest.