Change Lab Seats

Grow or shrink a PAID lab's seat count. Owner-only.

• Grow (seats > current cap): a prorated top-up order for the added seats over the CURRENT subscription's remaining days. The added seats co-terminate with that sub (no expiry extension) — the paid webhook lifts seat_quota to the new total. Returns a pay_url.
• Shrink (seats < current cap, ≥ active members ≥ min): takes effect immediately; surplus is NOT refunded (不退不换). Returns the lab.