Persists the highest step a user reached in the rebuilt /onboarding flow
so closing the tab mid-flow no longer wipes progress. Nullable Int
default 0; encoding mirrors the new wizard:
0 welcome / 1 goals / 2 source / 3 baseline / 4 done
Migration is idempotent (ADD COLUMN IF NOT EXISTS) so it tolerates
hand-applied dev-DB state without diverging from the migration history.
