Add COUNTER type with auto-increment and IsNegative field

COUNTER type:
- Only accepts integer values (rejects decimals)
- Auto-increment behavior: each mark adds to existing value
- Supports decrement via negative values (e.g., -2)
- Enforces minimum value of 0 (never negative)
- Default increment is 1 if no value provided

VALUE type:
- Accepts decimal values
- Replaces value (no auto-increment)

IsNegative field:
- New boolean field on Habit entity
- Persisted in database (is_negative column)
- Available in all DTOs and HTTP responses
- Marks habits as "negative" (e.g., candy consumption)

Examples:
- COUNTER: 5 + (-2) = 3, 2 + (-3) = 0, 0 + (-1) = 0
- VALUE: 5000 → 12000 = 12000 (replacement)

internal/domain/entities/habit.go

@@ -16,6 +16,7 @@ type Habit struct {
 	SpecificDays  []int
 	SpecificDates []int
 	CarryOver     bool
+	IsNegative    bool
 	TargetValue   *float64
 	CreatedAt     time.Time
 	ArchivedAt    *time.Time
@@ -27,14 +28,16 @@ func NewHabit(
 	habitType value_objects.HabitType,
 	frequency value_objects.Frequency,
 	carryOver bool,
+	isNegative bool,
 ) *Habit {
 	return &Habit{
-		UserID:    userID,
-		Name:      name,
-		Type:      habitType,
-		Frequency: frequency,
-		CarryOver: carryOver,
-		CreatedAt: time.Now(),
+		UserID:     userID,
+		Name:       name,
+		Type:       habitType,
+		Frequency:  frequency,
+		CarryOver:  carryOver,
+		IsNegative: isNegative,
+		CreatedAt:  time.Now(),
 	}
 }
 

internal/domain/entities/habit_test.go

@@ -13,8 +13,9 @@ func TestNewHabit(t *testing.T) {
 	habitType := value_objects.HabitTypeBoolean
 	frequency := value_objects.FrequencyDaily
 	carryOver := false
+	isNegative := false
 
-	habit := NewHabit(userID, name, habitType, frequency, carryOver)
+	habit := NewHabit(userID, name, habitType, frequency, carryOver, isNegative)
 
 	if habit.UserID != userID {
 		t.Errorf("Expected UserID %s, got %s", userID, habit.UserID)
@@ -46,7 +47,7 @@ func TestNewHabit(t *testing.T) {
 }
 
 func TestHabit_Archive(t *testing.T) {
-	habit := NewHabit("user-123", "Test Habit", value_objects.HabitTypeBoolean, value_objects.FrequencyDaily, false)
+	habit := NewHabit("user-123", "Test Habit", value_objects.HabitTypeBoolean, value_objects.FrequencyDaily, false, false)
 
 	if habit.ArchivedAt != nil {
 		t.Error("New habit should not be archived")
@@ -64,7 +65,7 @@ func TestHabit_Archive(t *testing.T) {
 }
 
 func TestHabit_IsActive(t *testing.T) {
-	habit := NewHabit("user-123", "Test Habit", value_objects.HabitTypeBoolean, value_objects.FrequencyDaily, false)
+	habit := NewHabit("user-123", "Test Habit", value_objects.HabitTypeBoolean, value_objects.FrequencyDaily, false, false)
 
 	if !habit.IsActive() {
 		t.Error("New habit should be active")
@@ -78,7 +79,7 @@ func TestHabit_IsActive(t *testing.T) {
 }
 
 func TestHabit_WithSpecificDays(t *testing.T) {
-	habit := NewHabit("user-123", "Workout", value_objects.HabitTypeBoolean, value_objects.FrequencyWeekly, false)
+	habit := NewHabit("user-123", "Workout", value_objects.HabitTypeBoolean, value_objects.FrequencyWeekly, false, false)
 	habit.SpecificDays = []int{1, 3, 5} // Monday, Wednesday, Friday
 
 	if len(habit.SpecificDays) != 3 {
@@ -91,7 +92,7 @@ func TestHabit_WithSpecificDays(t *testing.T) {
 }
 
 func TestHabit_WithTargetValue(t *testing.T) {
-	habit := NewHabit("user-123", "Drink Water", value_objects.HabitTypeCounter, value_objects.FrequencyDaily, false)
+	habit := NewHabit("user-123", "Drink Water", value_objects.HabitTypeCounter, value_objects.FrequencyDaily, false, false)
 	targetValue := 8.0
 	habit.TargetValue = &targetValue