refactor: introduce internal/domain/ layer, rename model.go → entity.go

Move all business-logic packages from internal/ root into internal/domain/:
  auth, cuisine, diary, dish, home, ingredient, language, menu, product,
  recipe, recognition, recommendation, savedrecipe, tag, units, user

Rename model.go → entity.go in packages that hold domain entities:
  diary, dish, home, ingredient, menu, product, recipe, savedrecipe, user

Update all import paths accordingly (adapters, infra/server, cmd/server,
tests). No logic changes.

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

backend/internal/domain/ingredient/entity.go

@@ -0,0 +1,29 @@
+package ingredient
+
+import (
+	"encoding/json"
+	"time"
+)
+
+// IngredientMapping is the canonical ingredient record used to link
+// 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"`
+	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"`
+	ProteinPer100g  *float64 `json:"protein_per_100g"`
+	FatPer100g      *float64 `json:"fat_per_100g"`
+	CarbsPer100g    *float64 `json:"carbs_per_100g"`
+	FiberPer100g    *float64 `json:"fiber_per_100g"`
+
+	StorageDays *int      `json:"storage_days"`
+	CreatedAt   time.Time `json:"created_at"`
+	UpdatedAt   time.Time `json:"updated_at"`
+}