Add domain layer tests (TDD - Red phase)

Following TDD principles, adding tests before implementation:

- Add HabitType value object tests (validation)
- Add Frequency value object tests (validation)
- Add User entity tests (creation, default timezone)
- Add Habit entity tests (creation, archive, active status)
- Add HabitEntry entity tests (creation, soft delete)

Tests cover core domain logic and business rules.
All tests will fail until implementation is added.

internal/domain/entities/habit_test.go

@@ -0,0 +1,105 @@
+package entities
+
+import (
+	"testing"
+	"time"
+
+	"habit-tracker-api/internal/domain/value_objects"
+)
+
+func TestNewHabit(t *testing.T) {
+	userID := "user-123"
+	name := "Morning Exercise"
+	habitType := value_objects.HabitTypeBoolean
+	frequency := value_objects.FrequencyDaily
+	carryOver := false
+
+	habit := NewHabit(userID, name, habitType, frequency, carryOver)
+
+	if habit.UserID != userID {
+		t.Errorf("Expected UserID %s, got %s", userID, habit.UserID)
+	}
+
+	if habit.Name != name {
+		t.Errorf("Expected Name %s, got %s", name, habit.Name)
+	}
+
+	if habit.Type != habitType {
+		t.Errorf("Expected Type %s, got %s", habitType, habit.Type)
+	}
+
+	if habit.Frequency != frequency {
+		t.Errorf("Expected Frequency %s, got %s", frequency, habit.Frequency)
+	}
+
+	if habit.CarryOver != carryOver {
+		t.Errorf("Expected CarryOver %v, got %v", carryOver, habit.CarryOver)
+	}
+
+	if habit.CreatedAt.IsZero() {
+		t.Error("CreatedAt should not be zero")
+	}
+
+	if habit.ArchivedAt != nil {
+		t.Error("ArchivedAt should be nil for new habit")
+	}
+}
+
+func TestHabit_Archive(t *testing.T) {
+	habit := NewHabit("user-123", "Test Habit", value_objects.HabitTypeBoolean, value_objects.FrequencyDaily, false)
+
+	if habit.ArchivedAt != nil {
+		t.Error("New habit should not be archived")
+	}
+
+	habit.Archive()
+
+	if habit.ArchivedAt == nil {
+		t.Error("Habit should be archived after calling Archive()")
+	}
+
+	if habit.ArchivedAt.After(time.Now()) {
+		t.Error("ArchivedAt should not be in the future")
+	}
+}
+
+func TestHabit_IsActive(t *testing.T) {
+	habit := NewHabit("user-123", "Test Habit", value_objects.HabitTypeBoolean, value_objects.FrequencyDaily, false)
+
+	if !habit.IsActive() {
+		t.Error("New habit should be active")
+	}
+
+	habit.Archive()
+
+	if habit.IsActive() {
+		t.Error("Archived habit should not be active")
+	}
+}
+
+func TestHabit_WithSpecificDays(t *testing.T) {
+	habit := NewHabit("user-123", "Workout", value_objects.HabitTypeBoolean, value_objects.FrequencyWeekly, false)
+	habit.SpecificDays = []int{1, 3, 5} // Monday, Wednesday, Friday
+
+	if len(habit.SpecificDays) != 3 {
+		t.Errorf("Expected 3 specific days, got %d", len(habit.SpecificDays))
+	}
+
+	if habit.SpecificDays[0] != 1 || habit.SpecificDays[1] != 3 || habit.SpecificDays[2] != 5 {
+		t.Errorf("Expected days [1,3,5], got %v", habit.SpecificDays)
+	}
+}
+
+func TestHabit_WithTargetValue(t *testing.T) {
+	habit := NewHabit("user-123", "Drink Water", value_objects.HabitTypeCounter, value_objects.FrequencyDaily, false)
+	targetValue := 8.0
+	habit.TargetValue = &targetValue
+
+	if habit.TargetValue == nil {
+		t.Error("TargetValue should not be nil")
+	}
+
+	if *habit.TargetValue != 8.0 {
+		t.Errorf("Expected TargetValue 8.0, got %f", *habit.TargetValue)
+	}
+}