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_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,