Add schedules[].unitsPerDose to the create/update contract with the
same whole-number-or-curated-fraction validation as the medication
level, built into the Prisma data field-by-field on both the create
and the schedule-replace path. Unwrap the nullable Decimal to a JSON
number at every medication read/write response through one shared
serializer so the wire shape cannot drift, keeping NULL as NULL to
mean inherit-the-medication-level.