EHBO and Accreditatiebalie shifts referenced sub-event time slots
(Vrijdag Early, Zaterdag Dag, etc.) but these cross_event sections
belong to the parent festival. The dropdown fetches time slots for
the festival event, so the sub-event time slot IDs had no matching
items — causing raw ULIDs in the dropdown and validation failures.
Added festival-level VOLUNTEER time slots mirroring the sub-event
schedule, and pointed cross_event section shifts at those instead.
Verified: all 81 shifts now reference time slots from the same event
as their section (zero mismatches).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
bert.hausmans
cb16cf9091