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/calories.go

@@ -0,0 +1,71 @@
+package user
+
+import (
+	"math"
+	"time"
+)
+
+// Activity level multipliers (Mifflin-St Jeor).
+var activityMultiplier = map[string]float64{
+	"low":      1.375,
+	"moderate": 1.55,
+	"high":     1.725,
+}
+
+// Goal adjustments in kcal.
+var goalAdjustment = map[string]float64{
+	"lose":     -500,
+	"maintain": 0,
+	"gain":     300,
+}
+
+// AgeFromDOB computes age in years from a "YYYY-MM-DD" string. Returns nil on error.
+func AgeFromDOB(dob *string) *int {
+	if dob == nil {
+		return nil
+	}
+	t, err := time.Parse("2006-01-02", *dob)
+	if err != nil {
+		return nil
+	}
+	now := time.Now()
+	years := now.Year() - t.Year()
+	if now.Month() < t.Month() || (now.Month() == t.Month() && now.Day() < t.Day()) {
+		years--
+	}
+	return &years
+}
+
+// CalculateDailyCalories computes the daily calorie target using the
+// Mifflin-St Jeor equation. Returns nil if any required parameter is missing.
+func CalculateDailyCalories(heightCM *int, weightKG *float64, age *int, gender, activity, goal *string) *int {
+	if heightCM == nil || weightKG == nil || age == nil || gender == nil || activity == nil || goal == nil {
+		return nil
+	}
+
+	// BMR = 10 * weight(kg) + 6.25 * height(cm) - 5 * age
+	bmr := 10**weightKG + 6.25*float64(*heightCM) - 5*float64(*age)
+
+	switch *gender {
+	case "male":
+		bmr += 5
+	case "female":
+		bmr -= 161
+	default:
+		return nil
+	}
+
+	mult, ok := activityMultiplier[*activity]
+	if !ok {
+		return nil
+	}
+
+	adj, ok := goalAdjustment[*goal]
+	if !ok {
+		return nil
+	}
+
+	tdee := bmr * mult
+	result := int(math.Round(tdee + adj))
+	return &result
+}