Fix code review items: atomic qty increment, unit updatedAt, suppliers/all endpoint, SKU unique index

packages/backend/src/db/migrations/0038_product_sku_unique.sql

@@ -0,0 +1,2 @@
+-- Add unique index on products.sku (null values are excluded from uniqueness)
+CREATE UNIQUE INDEX IF NOT EXISTS products_sku_unique ON product (sku) WHERE sku IS NOT NULL;