fix: add missing index for productId in PantryItem table migration

backend/prisma/migrations/20260422130000_user_scope_pantry_meal_plan/migration.sql

@@ -5,6 +5,7 @@ UPDATE `PantryItem`
 SET `userId` = (SELECT `id` FROM `User` ORDER BY `id` ASC LIMIT 1)
 WHERE `userId` IS NULL;
 
+ALTER TABLE `PantryItem` ADD INDEX `PantryItem_productId_idx`(`productId`);
 ALTER TABLE `PantryItem` DROP INDEX `PantryItem_productId_key`;
 ALTER TABLE `PantryItem` MODIFY `userId` INTEGER NOT NULL;
 ALTER TABLE `PantryItem` ADD INDEX `PantryItem_userId_idx`(`userId`);