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/menu/entity.go

@@ -0,0 +1,56 @@
+package menu
+
+// MenuPlan is a weekly meal plan for a user.
+type MenuPlan struct {
+	ID        string    `json:"id"`
+	WeekStart string    `json:"week_start"` // YYYY-MM-DD (Monday)
+	Days      []MenuDay `json:"days"`
+}
+
+// MenuDay groups three meal slots for one calendar day.
+type MenuDay struct {
+	Day           int        `json:"day"` // 1=Monday … 7=Sunday
+	Date          string     `json:"date"`
+	Meals         []MealSlot `json:"meals"`
+	TotalCalories float64    `json:"total_calories"`
+}
+
+// MealSlot holds a single meal within a day.
+type MealSlot struct {
+	ID       string      `json:"id"`
+	MealType string      `json:"meal_type"` // breakfast | lunch | dinner
+	Recipe   *MenuRecipe `json:"recipe,omitempty"`
+}
+
+// MenuRecipe is a thin projection of a saved recipe used in the menu view.
+type MenuRecipe struct {
+	ID        string        `json:"id"`
+	Title     string        `json:"title"`
+	ImageURL  string        `json:"image_url"`
+	Nutrition NutritionInfo `json:"nutrition_per_serving"`
+}
+
+// NutritionInfo holds macronutrient data.
+type NutritionInfo struct {
+	Calories float64 `json:"calories"`
+	ProteinG float64 `json:"protein_g"`
+	FatG     float64 `json:"fat_g"`
+	CarbsG   float64 `json:"carbs_g"`
+}
+
+// PlanItem is the input needed to create one menu_items row.
+type PlanItem struct {
+	DayOfWeek int
+	MealType  string
+	RecipeID  string
+}
+
+// ShoppingItem is one entry in the shopping list.
+type ShoppingItem struct {
+	Name     string  `json:"name"`
+	Category string  `json:"category"`
+	Amount   float64 `json:"amount"`
+	Unit     string  `json:"unit"`
+	Checked  bool    `json:"checked"`
+	InStock  float64 `json:"in_stock"`
+}