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>

src/db/schema.ts

@@ -21,6 +21,7 @@ export const items = sqliteTable("items", {
 	productUrl: text("product_url"),
 	imageFilename: text("image_filename"),
 	imageSourceUrl: text("image_source_url"),
+	quantity: integer("quantity").notNull().default(1),
 	createdAt: integer("created_at", { mode: "timestamp" })
 		.notNull()
 		.$defaultFn(() => new Date()),