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/recipe/entity.go

@@ -0,0 +1,49 @@
+package recipe
+
+import "time"
+
+// Recipe is a cooking variant of a Dish in the catalog.
+// It links to a Dish for all presentational data (name, image, cuisine, tags).
+type Recipe struct {
+	ID     string `json:"id"`
+	DishID string `json:"dish_id"`
+	Source string `json:"source"` // ai | user | spoonacular
+
+	Difficulty  *string `json:"difficulty"`
+	PrepTimeMin *int    `json:"prep_time_min"`
+	CookTimeMin *int    `json:"cook_time_min"`
+	Servings    *int    `json:"servings"`
+
+	CaloriesPerServing *float64 `json:"calories_per_serving"`
+	ProteinPerServing  *float64 `json:"protein_per_serving"`
+	FatPerServing      *float64 `json:"fat_per_serving"`
+	CarbsPerServing    *float64 `json:"carbs_per_serving"`
+	FiberPerServing    *float64 `json:"fiber_per_serving"`
+
+	Ingredients []RecipeIngredient `json:"ingredients"`
+	Steps       []RecipeStep       `json:"steps"`
+	Notes       *string            `json:"notes,omitempty"`
+
+	CreatedAt time.Time `json:"created_at"`
+	UpdatedAt time.Time `json:"updated_at"`
+}
+
+// RecipeIngredient is a single ingredient row from recipe_ingredients.
+type RecipeIngredient struct {
+	ID           string   `json:"id"`
+	IngredientID *string  `json:"ingredient_id"`
+	Name         string   `json:"name"`
+	Amount       float64  `json:"amount"`
+	UnitCode     *string  `json:"unit_code"`
+	IsOptional   bool     `json:"is_optional"`
+	SortOrder    int      `json:"sort_order"`
+}
+
+// RecipeStep is a single step row from recipe_steps.
+type RecipeStep struct {
+	ID           string  `json:"id"`
+	StepNumber   int     `json:"step_number"`
+	Description  string  `json:"description"`
+	TimerSeconds *int    `json:"timer_seconds"`
+	ImageURL     *string `json:"image_url"`
+}