feat: implement user-specific inventory management with security checks

backend/prisma/schema.prisma

@@ -25,6 +25,7 @@ model User {
   ownedRecipes Recipe[]      @relation("RecipeOwner")
   sharedRecipes RecipeShare[]
   ownedProducts Product[]
+  inventoryItems InventoryItem[]
   pantryItems PantryItem[]
   mealPlanEntries MealPlanEntry[]
   receiptAliases  ReceiptAlias[]
@@ -91,6 +92,7 @@ model UserProduct {
 
 model InventoryItem {
   id             Int      @id @default(autoincrement())
+  userId         Int
   productId      Int
   quantity       Decimal  @db.Decimal(10, 2)
   unit           String
@@ -107,9 +109,11 @@ model InventoryItem {
   createdAt      DateTime @default(now())
   updatedAt      DateTime @updatedAt
 
+  user    User    @relation(fields: [userId], references: [id], onDelete: Cascade)
   product Product @relation(fields: [productId], references: [id], onDelete: Cascade)
   consumptions   InventoryConsumption[]
 
+  @@index([userId])
   @@index([productId])
 }