Add shop catalog migration and atomic purchase/equip RPCs.

Upsert shop_items on deploy so the shop is never empty, and move buy/equip logic into Postgres for transactional coin deduction.

supabase/seed.sql

@@ -1,13 +1,15 @@
 -- Character layer styles sold in the shop (id = variant id, type = layer id).
 
 delete from public.user_items
-where item_id in ('hat_blue', 'glasses_round', 'room_sunset');
+where item_id in ('hat_blue', 'glasses_round', 'room_sunset', 'room-space', 'room-neon');
 
 delete from public.shop_items
 where id in (
   'hat_blue',
   'glasses_round',
   'room_sunset',
+  'room-space',
+  'room-neon',
   'pants-1',
   'pants-2',
   'shoes-1',