feat: enhance product model with subcategory, brand, tags, and nutrition; update related DTOs and services

backend/prisma/schema.prisma

@@ -12,6 +12,8 @@ model Product {
   name           String
   normalizedName String          @unique
   category       String?
+  subcategory    String?
+  brand          String?
   canonicalName  String?
   isActive       Boolean         @default(true)
   deletedAt      DateTime?
@@ -22,6 +24,8 @@ model Product {
   recipeIngredients RecipeIngredient[]
   pantryItems       PantryItem[]
   receiptAliases    ReceiptAlias[]
+  tags              ProductTag[]
+  nutrition         Nutrition?
 }
 
 model InventoryItem {
@@ -115,4 +119,33 @@ model MealPlanEntry {
 
   @@unique([date])
   @@index([date])
+}
+
+model Tag {
+  id       Int          @id @default(autoincrement())
+  name     String       @unique
+  products ProductTag[]
+}
+
+model ProductTag {
+  productId Int
+  tagId     Int
+  product   Product @relation(fields: [productId], references: [id], onDelete: Cascade)
+  tag       Tag     @relation(fields: [tagId], references: [id], onDelete: Cascade)
+
+  @@id([productId, tagId])
+  @@index([tagId])
+}
+
+model Nutrition {
+  id            Int     @id @default(autoincrement())
+  productId     Int     @unique
+  calories      Float?
+  protein       Float?
+  fat           Float?
+  carbohydrates Float?
+  salt          Float?
+  sugar         Float?
+  fiber         Float?
+  product       Product @relation(fields: [productId], references: [id], onDelete: Cascade)
 }