add abandoned status to distinguish given-up habits from completed ones

Completed and abandoned habits were both lumped under a single terminal
state, making it impossible to tell "finished successfully" apart from
"gave up" when reviewing habit history.

app/services/habit_service.py

@@ -79,9 +79,18 @@ def complete_habit(db: Session, habit: Habit) -> Habit:
     return habit
 
 
+def abandon_habit(db: Session, habit: Habit) -> Habit:
+    habit.status = HabitStatus.ABANDONED
+    habit.abandoned_at = datetime.now()
+    db.commit()
+    db.refresh(habit)
+    return habit
+
+
 def reactivate_habit(db: Session, habit: Habit) -> Habit:
     habit.status = HabitStatus.ACTIVE
     habit.completed_at = None
+    habit.abandoned_at = None
     db.commit()
     db.refresh(habit)
     return habit