Add isUsed on store items with API list filters.

Persists used/second-hand status per listing and supports isUsed=true on business and public store-item queries.

Co-authored-by: Cursor <cursoragent@cursor.com>

database/migrations/094_store_item_is_used.sql

@@ -0,0 +1,6 @@
+-- Used / second-hand flag on store listings (product-level).
+ALTER TABLE store_items
+  ADD COLUMN IF NOT EXISTS is_used BOOLEAN NOT NULL DEFAULT FALSE;
+
+CREATE INDEX IF NOT EXISTS idx_store_items_business_is_used
+  ON store_items (business_id, is_used);