Resolves a draft weekly schedule (start_dow/start_time → end_dow/end_time, day 0 = Monday) against the shop's timezone and returns the next concrete window it would produce — so a caller can confirm the window before creating a template.

Shop-scoped, NOT campaign-nested: the create flow has no campaign yet when the schedule is being drafted. The shop comes from x-shop-id; a shop_id in the body is ignored.

Writes nothing, but gated on read_write scope as the first step of the micro-challenge create flow. No Idempotency-Key needed.