Improve type safety with proper value objects for HabitType and Frequency

Replace generic string types with strongly-typed value objects throughout
the application layer. This change ensures compile-time type checking and
automatic validation during JSON deserialization.

Changes:
- Add JSON marshaling/unmarshaling to HabitType and Frequency value objects
- Update all DTOs to use typed fields instead of strings
- Update commands and queries to use proper types
- Remove unnecessary string conversions
- Add comprehensive JSON serialization tests
- Fix existing tests to work with typed fields

Benefits:
- Type safety: compiler catches invalid usage
- Automatic validation: invalid values rejected during JSON parsing
- Better code documentation and self-explanatory APIs
- Reduced runtime errors

internal/application/commands/create_habit.go

@@ -13,8 +13,8 @@ type CreateHabitCommand struct {
 	UserID        string
 	Name          string
 	Description   string
-	Type          string
-	Frequency     string
+	Type          value_objects.HabitType
+	Frequency     value_objects.Frequency
 	SpecificDays  []int
 	SpecificDates []int
 	CarryOver     bool
@@ -30,25 +30,23 @@ func NewCreateHabitHandler(habitRepo repositories.HabitRepository) *CreateHabitH
 }
 
 func (h *CreateHabitHandler) Handle(ctx context.Context, cmd CreateHabitCommand) (string, error) {
-	habitType := value_objects.HabitType(cmd.Type)
-	if !habitType.IsValid() {
+	if !cmd.Type.IsValid() {
 		return "", errors.ErrInvalidInput
 	}
 
-	frequency := value_objects.Frequency(cmd.Frequency)
-	if !frequency.IsValid() {
+	if !cmd.Frequency.IsValid() {
 		return "", errors.ErrInvalidInput
 	}
 
-	if frequency == value_objects.FrequencyWeekly && len(cmd.SpecificDays) == 0 {
+	if cmd.Frequency == value_objects.FrequencyWeekly && len(cmd.SpecificDays) == 0 {
 		return "", errors.ErrInvalidInput
 	}
 
-	if frequency == value_objects.FrequencyMonthly && len(cmd.SpecificDates) == 0 {
+	if cmd.Frequency == value_objects.FrequencyMonthly && len(cmd.SpecificDates) == 0 {
 		return "", errors.ErrInvalidInput
 	}
 
-	habit := entities.NewHabit(cmd.UserID, cmd.Name, habitType, frequency, cmd.CarryOver)
+	habit := entities.NewHabit(cmd.UserID, cmd.Name, cmd.Type, cmd.Frequency, cmd.CarryOver)
 	habit.Description = cmd.Description
 	habit.SpecificDays = cmd.SpecificDays
 	habit.SpecificDates = cmd.SpecificDates