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>

tests/test_schemas_habit.py

@@ -1,7 +1,7 @@
 import pytest
 from pydantic import ValidationError
 
-from app.models.habit import ALL_WEEKDAYS_MASK, HabitType
+from app.models.habit import ALL_WEEKDAYS_MASK, HabitDifficulty, HabitType
 from app.schemas.habit import HabitCreate
 
 
@@ -49,3 +49,18 @@ def test_weekdays_mask_single_day_accepted():
 def test_weekdays_mask_defaults_to_all_days():
     habit = HabitCreate(**_base_kwargs())
     assert habit.weekdays_mask == ALL_WEEKDAYS_MASK
+
+
+def test_difficulty_defaults_to_medium():
+    habit = HabitCreate(**_base_kwargs())
+    assert habit.difficulty == HabitDifficulty.MEDIUM
+
+
+def test_difficulty_accepts_explicit_value():
+    habit = HabitCreate(**_base_kwargs(difficulty=HabitDifficulty.UNLIMITED))
+    assert habit.difficulty == HabitDifficulty.UNLIMITED
+
+
+def test_difficulty_rejects_invalid_value():
+    with pytest.raises(ValidationError):
+        HabitCreate(**_base_kwargs(difficulty="impossible"))