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/schemas/habit.py

@@ -2,13 +2,14 @@ from datetime import datetime, time
 
 from pydantic import BaseModel, ConfigDict, field_validator
 
-from app.models.habit import ALL_WEEKDAYS_MASK, HabitStatus, HabitType
+from app.models.habit import ALL_WEEKDAYS_MASK, HabitDifficulty, HabitStatus, HabitType
 
 
 class HabitBase(BaseModel):
     name: str
     habit_type: HabitType
     weekdays_mask: int = ALL_WEEKDAYS_MASK
+    difficulty: HabitDifficulty = HabitDifficulty.MEDIUM
     condition_text: str | None = None
     reminder_time: time | None = None
 
@@ -52,6 +53,7 @@ class HabitOut(BaseModel):
     habit_type: HabitType
     status: HabitStatus
     weekdays_mask: int
+    difficulty: HabitDifficulty
     condition_text: str | None
     reminder_time: time | None
     created_at: datetime