Configure the training public registration form (full replacement).

• The returned URL is derived from the training immutable inscription hash and never changes across calls.
• firstName and lastName are always displayed and mandatory and cannot be changed.
• fields and enrollment are required: resetting the form is always explicit ({ "fields": [], "enrollment": {} }), an empty body is rejected.
• Any configurable field omitted from fields defaults to displayed: false, mandatory: false.
• Every field provided in fields must state displayed and mandatory explicitly.
• mandatory: true requires displayed: true.
• Archived trainings are rejected with ARCHIVED_TRAINING_EXISTS.
• Custom-variable keys must reference existing student custom variables; they are never created here.