feat: add servings field to Recipe model and implement inventory comparison functionality

backend/prisma/schema.prisma

@@ -72,6 +72,7 @@ model Recipe {
   description  String?            @db.Text
   instructions String?            @db.Text
   imageUrl     String?
+  servings     Int?
   createdAt    DateTime           @default(now())
   updatedAt    DateTime           @updatedAt