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/queries/get_habit_by_id.go

@@ -35,8 +35,8 @@ func (h *GetHabitByIDHandler) Handle(ctx context.Context, query GetHabitByIDQuer
 	return &HabitDTO{
 		ID:           habit.ID,
 		Name:         habit.Name,
-		Type:         string(habit.Type),
-		Frequency:    string(habit.Frequency),
+		Type:         habit.Type,
+		Frequency:    habit.Frequency,
 		TargetValue:  habit.TargetValue,
 		CarryOver:    habit.CarryOver,
 		SpecificDays: habit.SpecificDays,

internal/application/queries/get_habit_by_id_test.go

@@ -53,7 +53,7 @@ func TestGetHabitByIDHandler_ReturnsHabitSuccessfully(t *testing.T) {
 		t.Errorf("Expected name 'Drink Water', got %s", result.Name)
 	}
 
-	if result.Type != string(value_objects.HabitTypeValue) {
+	if result.Type != value_objects.HabitTypeValue {
 		t.Errorf("Expected type %s, got %s", value_objects.HabitTypeValue, result.Type)
 	}
 

internal/application/queries/get_todays_habits.go

@@ -5,13 +5,14 @@ import (
 	"time"
 
 	"apocapoc-api/internal/domain/repositories"
+	"apocapoc-api/internal/domain/value_objects"
 	"apocapoc-api/internal/shared/utils"
 )
 
 type TodaysHabitDTO struct {
 	ID            string
 	Name          string
-	Type          string
+	Type          value_objects.HabitType
 	TargetValue   *float64
 	ScheduledDate time.Time
 	IsCarriedOver bool
@@ -80,7 +81,7 @@ func (h *GetTodaysHabitsHandler) Handle(
 			result = append(result, TodaysHabitDTO{
 				ID:            habit.ID,
 				Name:          habit.Name,
-				Type:          string(habit.Type),
+				Type:          habit.Type,
 				TargetValue:   habit.TargetValue,
 				ScheduledDate: query.Date,
 				IsCarriedOver: !shouldAppear && habit.CarryOver,

internal/application/queries/get_user_habits.go

@@ -4,13 +4,14 @@ import (
 	"context"
 
 	"apocapoc-api/internal/domain/repositories"
+	"apocapoc-api/internal/domain/value_objects"
 )
 
 type HabitDTO struct {
 	ID           string
 	Name         string
-	Type         string
-	Frequency    string
+	Type         value_objects.HabitType
+	Frequency    value_objects.Frequency
 	TargetValue  *float64
 	CarryOver    bool
 	SpecificDays []int
@@ -41,8 +42,8 @@ func (h *GetUserHabitsHandler) Handle(ctx context.Context, query GetUserHabitsQu
 		result = append(result, HabitDTO{
 			ID:           habit.ID,
 			Name:         habit.Name,
-			Type:         string(habit.Type),
-			Frequency:    string(habit.Frequency),
+			Type:         habit.Type,
+			Frequency:    habit.Frequency,
 			TargetValue:  habit.TargetValue,
 			CarryOver:    habit.CarryOver,
 			SpecificDays: habit.SpecificDays,

internal/application/queries/get_user_habits_test.go

@@ -92,11 +92,11 @@ func TestGetUserHabitsHandler_IncludesAllHabitFields(t *testing.T) {
 		t.Errorf("Expected name 'Drink Water', got %s", result.Name)
 	}
 
-	if result.Type != string(value_objects.HabitTypeValue) {
+	if result.Type != value_objects.HabitTypeValue {
 		t.Errorf("Expected type %s, got %s", value_objects.HabitTypeValue, result.Type)
 	}
 
-	if result.Frequency != string(value_objects.FrequencyDaily) {
+	if result.Frequency != value_objects.FrequencyDaily {
 		t.Errorf("Expected frequency %s, got %s", value_objects.FrequencyDaily, result.Frequency)
 	}