Remove Seats

Remove extra seat add-ons from the existing subscription.

Reduces the seat add-on quantity by seats_to_remove. Cannot go below zero extra seats. If the quantity reaches zero the add-on line item is removed from the subscription entirely.

When the reduction would put the workspace over the new seat limit, the caller must supply member_ids_to_evict — exactly as many IDs as the overflow count. If they are not supplied, the endpoint returns 409 with type="member_eviction_required" and the full list of evictable members (non-owners, sorted newest-first) so the frontend can render a picker.

The Stripe item modification and the member removals are committed together so the workspace is never left in a partially-updated state.