Add a nullable units_per_dose to medication_schedules so a split-dose
plan can consume a different unit count at different times of day (a
whole tablet in the morning, a half at noon) on ONE medication instead
of two. NULL inherits the medication-level units_per_dose, so every
existing row keeps its current behaviour. Migration 0332 adds one
nullable Decimal(10,4) column with no backfill.
