feat: add support for alternative ingredients; implement JSON storage and parsing logic

backend/prisma/migrations/20260504220420_add_alternative_product_ids/migration.sql

@@ -0,0 +1 @@
+ALTER TABLE `RecipeIngredient` ADD COLUMN `alternativeProductIds` JSON NULL;

backend/prisma/schema.prisma

@@ -156,6 +156,7 @@ model RecipeIngredient {
   quantity  Decimal  @db.Decimal(10, 2)
   unit      String
   note      String?
+  alternativeProductIds Json?   // [id, id, ...] — alternativa produkter (t.ex. "ris eller couscous")
 
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt