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/home/entity.go

@@ -0,0 +1,39 @@
+package home
+
+// Summary is the response for GET /home/summary.
+type Summary struct {
+	Today           TodaySummary     `json:"today"`
+	ExpiringSoon    []ExpiringSoon   `json:"expiring_soon"`
+	Recommendations []Recommendation `json:"recommendations"`
+}
+
+// TodaySummary contains the day-level overview.
+type TodaySummary struct {
+	Date           string      `json:"date"`
+	DailyGoal      int         `json:"daily_goal"`
+	LoggedCalories float64     `json:"logged_calories"`
+	Plan           []MealPlan  `json:"plan"`
+}
+
+// MealPlan is a single planned meal slot for today.
+type MealPlan struct {
+	MealType       string   `json:"meal_type"`
+	RecipeTitle    *string  `json:"recipe_title"`
+	RecipeImageURL *string  `json:"recipe_image_url"`
+	Calories       *float64 `json:"calories"`
+}
+
+// ExpiringSoon is a product expiring within 3 days.
+type ExpiringSoon struct {
+	Name          string `json:"name"`
+	ExpiresInDays int    `json:"expires_in_days"`
+	Quantity      string `json:"quantity"`
+}
+
+// Recommendation is a saved recipe shown on the home screen.
+type Recommendation struct {
+	ID       string   `json:"id"`
+	Title    string   `json:"title"`
+	ImageURL string   `json:"image_url"`
+	Calories *float64 `json:"calories"`
+}