feat: add location field to PantryItem model and update related functionality

Co-authored-by: Copilot <copilot@github.com>

backend/prisma/migrations/20260506180000_add_pantry_location/migration.sql

@@ -0,0 +1,2 @@
+ALTER TABLE `PantryItem`
+ADD COLUMN `location` VARCHAR(191) NULL;

backend/prisma/schema.prisma

@@ -172,6 +172,7 @@ model PantryItem {
   id        Int      @id @default(autoincrement())
   userId    Int
   productId Int
+  location  String?
   user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)
   product   Product  @relation(fields: [productId], references: [id], onDelete: Cascade)
   createdAt DateTime @default(now())