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/infrastructure/http/habit_handlers.go

@@ -84,6 +84,7 @@ func (h *HabitHandlers) CreateHabit(w http.ResponseWriter, r *http.Request) {
 		SpecificDays:  req.SpecificDays,
 		SpecificDates: req.SpecificDates,
 		CarryOver:     req.CarryOver,
+		IsNegative:    req.IsNegative,
 		TargetValue:   req.TargetValue,
 	}
 
@@ -137,6 +138,7 @@ func (h *HabitHandlers) GetUserHabits(w http.ResponseWriter, r *http.Request) {
 			SpecificDays: habit.SpecificDays,
 			TargetValue:  habit.TargetValue,
 			CarryOver:    habit.CarryOver,
+			IsNegative:   habit.IsNegative,
 		}
 	}
 
@@ -192,6 +194,7 @@ func (h *HabitHandlers) GetHabitByID(w http.ResponseWriter, r *http.Request) {
 		SpecificDays: habit.SpecificDays,
 		TargetValue:  habit.TargetValue,
 		CarryOver:    habit.CarryOver,
+		IsNegative:   habit.IsNegative,
 	}
 
 	respondJSON(w, http.StatusOK, response)
@@ -464,6 +467,7 @@ func (h *HabitHandlers) GetTodaysHabits(w http.ResponseWriter, r *http.Request)
 			Name:          habit.Name,
 			Type:          habit.Type,
 			TargetValue:   habit.TargetValue,
+			IsNegative:    habit.IsNegative,
 			ScheduledDate: habit.ScheduledDate,
 			IsCarriedOver: habit.IsCarriedOver,
 		}