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/diary/entity.go

@@ -0,0 +1,37 @@
+package diary
+
+import "time"
+
+// Entry is a single meal diary record.
+type Entry struct {
+	ID        string    `json:"id"`
+	Date      string    `json:"date"` // YYYY-MM-DD
+	MealType  string    `json:"meal_type"`
+	Name      string    `json:"name"`
+	Portions  float64   `json:"portions"`
+	Calories  *float64  `json:"calories,omitempty"`
+	ProteinG  *float64  `json:"protein_g,omitempty"`
+	FatG      *float64  `json:"fat_g,omitempty"`
+	CarbsG    *float64  `json:"carbs_g,omitempty"`
+	Source    string    `json:"source"`
+	DishID    *string   `json:"dish_id,omitempty"`
+	RecipeID  *string   `json:"recipe_id,omitempty"`
+	PortionG  *float64  `json:"portion_g,omitempty"`
+	CreatedAt time.Time `json:"created_at"`
+}
+
+// CreateRequest is the body for POST /diary.
+type CreateRequest struct {
+	Date     string   `json:"date"`
+	MealType string   `json:"meal_type"`
+	Name     string   `json:"name"`
+	Portions float64  `json:"portions"`
+	Calories *float64 `json:"calories"`
+	ProteinG *float64 `json:"protein_g"`
+	FatG     *float64 `json:"fat_g"`
+	CarbsG   *float64 `json:"carbs_g"`
+	Source   string   `json:"source"`
+	DishID   *string  `json:"dish_id"`
+	RecipeID *string  `json:"recipe_id"`
+	PortionG *float64 `json:"portion_g"`
+}