Persist how far a village_free worker got through the guided walkthrough.

Server-side rather than local storage because the villager is expected to start on the website and finish in the app; a walkthrough that re-runs on the second device is exactly the friction this audience won't forgive.

Stored as one namespaced key on the onboarding_setup_prefs JSONB column. Monotonicity lives in SQL, not here: merge_village_onboarding_step takes GREATEST(stored, p_step) in the same statement as the write, so the row lock serialises it. A Python read-then-compare would be a TOCTOU - two devices (the expected web→app path) both read the old value, and the lower write lands last, dropping the villager back into a walkthrough they already skipped. The RPC refuses any p_user_id other than auth.uid(), so a caller can only ever move their own progress.

Returns the whole merged column, so a client hydrating its prefs snapshot from the response can't silently lose the other keys.