Create a room under a per-user advisory lock (limit, card and insert in one
transaction) and book cards under per-machine locks. A free plan with no model
is placed on an entry card only. Boxes with no card hold their machine's first
card; the new migration backfills single-GPU fleet boxes and adds
box.gpu_slot. Machine.Gpu accepts slot and renderNode; session.start carries
gpuSlot and gpuPciId. A stopped room re-checks its card when a run is
requested.