feat: add quantity field to items schema

Add integer quantity column (default 1) to the items table, generate
the corresponding Drizzle migration, and extend createItemSchema /
updateItemSchema with an optional positive-integer quantity field.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

drizzle/0008_loving_colossus.sql

@@ -0,0 +1 @@
+ALTER TABLE `items` ADD `quantity` integer DEFAULT 1 NOT NULL;