Store onboarding step in session

post/api/auth/onboarding-step

Request body

referral_sourcestring nullable

Referral source (how they heard about Scanopy)

referral_source_otherstring nullable

Free-text referral source (when "other" is selected)

stepstring required

use_casestring nullable

Use case selection (homelab, company, msp)

Step saved

dataTupleUnit — unresolved $ref

errorstring nullable

successboolean required