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_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,