refactor: migrate ingredient aliases/categories to dedicated tables, drop spoonacular_id

- migration 012: create ingredient_categories + ingredient_category_translations
  tables (7 slugs, Russian names); add ingredient_aliases (ingredient_id, lang,
  alias) with GIN trigram index; migrate aliases JSONB from ingredient_mappings
  and ingredient_translations; drop aliases columns and spoonacular_id; add
  UNIQUE (canonical_name) as conflict key
- ingredient/model: remove SpoonacularID, add CategoryName for localized display
- ingredient/repository: conflict on canonical_name; GetByID/Search join category
  translations and aliases lateral; new UpsertAliases (pgx batch),
  UpsertCategoryTranslation; remove GetBySpoonacularID; split scan helpers into
  scanMappingWrite / scanMappingRead
- gemini/recognition: add IngredientTranslation type; IngredientClassification
  now carries Translations []IngredientTranslation instead of CanonicalNameRu;
  update ClassifyIngredient prompt to English with structured translations array
- recognition/handler: update ingredientRepo interface; saveClassification uses
  UpsertAliases and iterates Translations
- recipe/model: remove SpoonacularID from RecipeIngredient
- integration tests: remove SpoonacularID fixtures, replace GetBySpoonacularID
  tests with GetByID, add UpsertAliases and UpsertCategoryTranslation tests
- Flutter: remove canonicalNameRu from IngredientMapping, add categoryName;
  displayName returns server-resolved canonicalName; regenerate .g.dart

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

backend/internal/ingredient/model.go

@@ -6,15 +6,15 @@ import (
 )
 
 // IngredientMapping is the canonical ingredient record used to link
-// user products, recipe ingredients, and Spoonacular data.
+// user products and recipe ingredients.
 // CanonicalName holds the content for the language resolved at query time
 // (English by default, or from ingredient_translations when available).
 type IngredientMapping struct {
 	ID            string          `json:"id"`
 	CanonicalName string          `json:"canonical_name"`
-	SpoonacularID *int            `json:"spoonacular_id"`
-	Aliases       json.RawMessage `json:"aliases"`    // []string
+	Aliases       json.RawMessage `json:"aliases"`       // []string, populated by read queries
 	Category      *string         `json:"category"`
+	CategoryName  *string         `json:"category_name"` // localized category display name
 	DefaultUnit   *string         `json:"default_unit"`
 
 	CaloriesPer100g *float64 `json:"calories_per_100g"`