Resolves the artist subject + event_id + engagement for the
artist_advance portal flow. Per RFC v0.2 D15 + ARCH-FORM-BUILDER
§17.3 footnote: master Artist is the subject (preserves
form_submissions.subject_type='artist'), engagement provides
event_id (per WS-4 denormalisation), and engagement itself rides
along so callers can resolve advance_section context without a
second query.
Token comparison uses SHA-256 hex digest matching Session 1's
storage shape (commit eb6d396). Two domain exceptions distinguish
404 (no matching token → InvalidPortalTokenException) from 410
(master artist soft-deleted post-engagement → ArtistDeletedException
with engagementId attached).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
bert.hausmans
cc48011da6