Currency, storefronts, purchasing (#12)

And a few other minor things, but primarily, the balance table exists and also consumable/inventory table.

apps/econ/migrations/0004_inventory.sql

@@ -0,0 +1,14 @@
+-- Owned avatar items, owned by the `econ` worker. One row per (account, item): the
+-- items a player has bought from a storefront. Granted at purchase time by
+-- `/api/storefronts/v2/buyItem` and read back by `/api/avatar/v4/items`, where they are
+-- concatenated with the default catalog. The item is keyed by its `AvatarItemDesc` (the
+-- gift-drop's item guid string) so re-buying the same item is a no-op rather than a
+-- duplicate row; `data` is the rendered avatar-item DTO. Kept in sync with
+-- INVENTORY_SCHEMA_DDL in src/inventory-db.ts.
+
+CREATE TABLE IF NOT EXISTS inventory (
+  account_id INTEGER NOT NULL,
+  avatar_item_desc TEXT NOT NULL,
+  data TEXT NOT NULL,
+  PRIMARY KEY (account_id, avatar_item_desc)
+  );