feat: implement user-specific inventory management with security checks

backend/prisma/migrations/20260507203000_inventory_item_user_scope/migration.sql

@@ -0,0 +1,18 @@
+ALTER TABLE `InventoryItem`
+  ADD COLUMN `userId` INT NULL;
+
+UPDATE `InventoryItem` AS ii
+JOIN `Product` AS p ON p.`id` = ii.`productId`
+SET ii.`userId` = p.`ownerId`
+WHERE ii.`userId` IS NULL;
+
+SET @fallback_user_id := (SELECT `id` FROM `User` ORDER BY `id` ASC LIMIT 1);
+UPDATE `InventoryItem`
+SET `userId` = @fallback_user_id
+WHERE `userId` IS NULL;
+
+ALTER TABLE `InventoryItem`
+  MODIFY COLUMN `userId` INT NOT NULL,
+  ADD INDEX `InventoryItem_userId_idx` (`userId`),
+  ADD CONSTRAINT `InventoryItem_userId_fkey`
+    FOREIGN KEY (`userId`) REFERENCES `User`(`id`) ON DELETE CASCADE ON UPDATE CASCADE;