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/user/entity.go

@@ -0,0 +1,44 @@
+package user
+
+import (
+	"encoding/json"
+	"time"
+)
+
+type User struct {
+	ID            string          `json:"id"`
+	FirebaseUID   string          `json:"-"`
+	Email         string          `json:"email"`
+	Name          string          `json:"name"`
+	AvatarURL     *string         `json:"avatar_url"`
+	HeightCM      *int            `json:"height_cm"`
+	WeightKG      *float64        `json:"weight_kg"`
+	DateOfBirth   *string         `json:"date_of_birth"`
+	Gender        *string         `json:"gender"`
+	Activity      *string         `json:"activity"`
+	Goal          *string         `json:"goal"`
+	DailyCalories *int            `json:"daily_calories"`
+	Plan          string          `json:"plan"`
+	Preferences   json.RawMessage `json:"preferences"`
+	CreatedAt     time.Time       `json:"created_at"`
+	UpdatedAt     time.Time       `json:"updated_at"`
+}
+
+type UpdateProfileRequest struct {
+	Name          *string          `json:"name"`
+	HeightCM      *int             `json:"height_cm"`
+	WeightKG      *float64         `json:"weight_kg"`
+	DateOfBirth   *string          `json:"date_of_birth"`
+	Gender        *string          `json:"gender"`
+	Activity      *string          `json:"activity"`
+	Goal          *string          `json:"goal"`
+	Preferences   *json.RawMessage `json:"preferences"`
+	DailyCalories *int             `json:"-"` // internal, set by service
+}
+
+// HasBodyParams returns true if any body parameter is being updated
+// that would require recalculation of daily calories.
+func (r *UpdateProfileRequest) HasBodyParams() bool {
+	return r.HeightCM != nil || r.WeightKG != nil || r.DateOfBirth != nil ||
+		r.Gender != nil || r.Activity != nil || r.Goal != nil
+}