Currency, storefronts, purchasing (#12)

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

apps/econ/migrations/0005_consumable.sql

@@ -0,0 +1,18 @@
+-- Owned consumables, owned by the `econ` worker. Unlike avatar items (own-once, one
+-- row per (account, item)), consumables stack: each purchase inserts a fresh instance
+-- row carrying its own id, count and created_at. Granted at purchase time by
+-- `/api/storefronts/v2/buyItem` (when the gift-drop carries a `ConsumableItemDesc`) and
+-- read back by `/api/consumables/v2/getUnlocked`, which groups a player's rows by
+-- `consumable_item_desc` into the client's unlocked-consumable DTO (its `Ids`/`CreatedAts`
+-- are these per-instance columns; `Count` their sum). Kept in sync with
+-- CONSUMABLE_SCHEMA_DDL in src/consumables-db.ts.
+
+CREATE TABLE IF NOT EXISTS consumable (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  account_id INTEGER NOT NULL,
+  consumable_item_desc TEXT NOT NULL,
+  count INTEGER NOT NULL,
+  created_at TEXT NOT NULL
+  );
+
+CREATE INDEX IF NOT EXISTS idx_consumable_account ON consumable (account_id);