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/routers/habits.py

@@ -69,6 +69,12 @@ def complete_habit(habit_id: int, db: Session = Depends(get_db), current_user: U
     return habit_service.complete_habit(db, habit)
 
 
+@router.post("/{habit_id}/abandon", response_model=HabitOut)
+def abandon_habit(habit_id: int, db: Session = Depends(get_db), current_user: User = Depends(require_login)):
+    habit = _get_habit_or_404(db, habit_id, current_user.id)
+    return habit_service.abandon_habit(db, habit)
+
+
 @router.post("/{habit_id}/reactivate", response_model=HabitOut)
 def reactivate_habit(habit_id: int, db: Session = Depends(get_db), current_user: User = Depends(require_login)):
     habit = _get_habit_or_404(db, habit_id, current_user.id)