Open a rental

Opens a rental in the active state: the unit leaves the store with the customer until dueAt, which the server DERIVES as startAt + days (do not send it). days is one of the four offered tiers — 1, 3, 7 or 14 — and no other value is accepted. MONEY, read carefully: this endpoint moves NONE. depositAmountCents / depositReference merely RECORD a caution the payment terminal has already pre-authorised (the money is HELD on the customer’s card, not captured), and the rental fee itself (ratePerDayCents x days) is NOT charged here or by any other endpoint in this API — take it at the till. Never call this before the pre-auth actually exists; you would open an unsecured rental. id is CALLER-SUPPLIED and is the dedup key: if your call times out, retry with the SAME id — a 409 CONFLICT then means your first attempt committed, so read it back with getRental instead of minting a new id. The duplicate-id check runs FIRST, so an existing id returns 409 even when the window is also invalid — you will not see the 422 until the id is unique. 422 when the derived dueAt is not in the future (a stale startAt, or a window that has already elapsed); 400 on a bad tier, a non-hex skuId or a non-positive amount.