feat: core schema redesign — dishes, structured recipes, cuisines, tags (iteration 7)

Replaces the flat JSONB-based recipe schema with a normalized relational model:

Schema (migrations consolidated to 001_initial_schema + 002_seed_data):
- New: dishes, dish_translations, dish_tags — canonical dish catalog
- New: cuisines, tags, dish_categories with _translations tables + full seed data
- New: recipe_ingredients, recipe_steps with _translations (replaces JSONB blobs)
- New: user_saved_recipes thin bookmark (drops saved_recipes + saved_recipe_translations)
- New: product_ingredients M2M table
- recipes: now a cooking variant of a dish (dish_id FK, no title/JSONB columns)
- recipe_translations: repurposed to per-language notes only
- products: mapping_id → primary_ingredient_id
- menu_items: recipe_id FK → recipes; adds dish_id
- meal_diary: adds dish_id, recipe_id → recipes, portion_g

Backend (Go):
- New packages: internal/cuisine, internal/tag, internal/dish (registry + handler + repo)
- New GET /cuisines, GET /tags (public), GET /dishes, GET /dishes/{id}, GET /recipes/{id}
- recipe, savedrecipe, menu, diary, product, ingredient packages updated for new schema

Flutter:
- New models: Cuisine, Tag; new providers: cuisineNamesProvider, tagNamesProvider
- recipe.dart: RecipeIngredient gains unit_code + effectiveUnit getter
- saved_recipe.dart: thin model, manual fromJson, computed nutrition getter
- diary_entry.dart: adds dishId, recipeId, portionG
- recipe_detail_screen.dart: localized cuisine/tag names via providers

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/product/model.go

@@ -4,28 +4,30 @@ import "time"
 
 // Product is a user's food item in their pantry.
 type Product struct {
-	ID           string    `json:"id"`
-	UserID       string    `json:"user_id"`
-	MappingID    *string   `json:"mapping_id"`
-	Name         string    `json:"name"`
-	Quantity     float64   `json:"quantity"`
-	Unit         string    `json:"unit"`
-	Category     *string   `json:"category"`
-	StorageDays  int       `json:"storage_days"`
-	AddedAt      time.Time `json:"added_at"`
-	ExpiresAt    time.Time `json:"expires_at"`
-	DaysLeft     int       `json:"days_left"`
-	ExpiringSoon bool      `json:"expiring_soon"`
+	ID                 string    `json:"id"`
+	UserID             string    `json:"user_id"`
+	PrimaryIngredientID *string  `json:"primary_ingredient_id"`
+	Name               string    `json:"name"`
+	Quantity           float64   `json:"quantity"`
+	Unit               string    `json:"unit"`
+	Category           *string   `json:"category"`
+	StorageDays        int       `json:"storage_days"`
+	AddedAt            time.Time `json:"added_at"`
+	ExpiresAt          time.Time `json:"expires_at"`
+	DaysLeft           int       `json:"days_left"`
+	ExpiringSoon       bool      `json:"expiring_soon"`
 }
 
 // CreateRequest is the body for POST /products.
 type CreateRequest struct {
-	MappingID   *string  `json:"mapping_id"`
-	Name        string   `json:"name"`
-	Quantity    float64  `json:"quantity"`
-	Unit        string   `json:"unit"`
-	Category    *string  `json:"category"`
-	StorageDays int      `json:"storage_days"`
+	PrimaryIngredientID *string  `json:"primary_ingredient_id"`
+	// Accept both "primary_ingredient_id" (new) and "mapping_id" (legacy client) fields.
+	MappingID           *string  `json:"mapping_id"`
+	Name                string   `json:"name"`
+	Quantity            float64  `json:"quantity"`
+	Unit                string   `json:"unit"`
+	Category            *string  `json:"category"`
+	StorageDays         int      `json:"storage_days"`
 }
 
 // UpdateRequest is the body for PUT /products/{id}.