add habit goal-period difficulty and habit formation research notes

Ground the 21-day habit myth in actual research (Lally et al. 2010) and let
users pick a target period (21/66/254 days or unlimited) matching that
study's easy/median/hard automaticity timelines when creating a habit,
with progress shown on the habits list.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/services/habit_service.py

@@ -45,6 +45,7 @@ def create_habit(db: Session, user_id: int, data: HabitCreate) -> Habit:
         name=data.name,
         habit_type=data.habit_type,
         weekdays_mask=data.weekdays_mask,
+        difficulty=data.difficulty,
         condition_text=data.condition_text,
         reminder_time=data.reminder_time,
         status=HabitStatus.ACTIVE,
@@ -59,6 +60,7 @@ def update_habit(db: Session, habit: Habit, data: HabitUpdate) -> Habit:
     habit.name = data.name
     habit.habit_type = data.habit_type
     habit.weekdays_mask = data.weekdays_mask
+    habit.difficulty = data.difficulty
     habit.condition_text = data.condition_text
     habit.reminder_time = data.reminder_time
     db.commit()