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/models/habit.py

@@ -1,5 +1,5 @@
 import enum
-from datetime import datetime, time
+from datetime import date, datetime, time, timedelta
 
 from sqlalchemy import Enum, ForeignKey, Integer, SmallInteger, String, Time
 from sqlalchemy.orm import Mapped, mapped_column, relationship
@@ -21,6 +21,29 @@ class HabitStatus(str, enum.Enum):
     ABANDONED = "abandoned"
 
 
+class HabitDifficulty(str, enum.Enum):
+    """목표 기간 난이도. Lally et al.(2010, UCL)의 습관 자동화 소요 기간 실증 연구를 근거로 한다
+    (자세한 내용은 habit-formation-research.md 참고).
+    - EASY: 기존 습관에 붙이기 쉬운 단순 행동이 자동화되는 데 걸린 기간(약 3주)
+    - MEDIUM: 전체 참가자의 자동화 소요 기간 중앙값
+    - HARD: 노력이 많이 드는 행동이 자동화되는 데 걸린 기간의 상한(연구 관찰 범위 18~254일 중 최대값)
+    - UNLIMITED: 목표 종료 시점 없이 계속 트래킹만 하는 습관
+    """
+
+    EASY = "easy"
+    MEDIUM = "medium"
+    HARD = "hard"
+    UNLIMITED = "unlimited"
+
+
+HABIT_DIFFICULTY_TARGET_DAYS: dict[HabitDifficulty, int | None] = {
+    HabitDifficulty.EASY: 21,
+    HabitDifficulty.MEDIUM: 66,
+    HabitDifficulty.HARD: 254,
+    HabitDifficulty.UNLIMITED: None,
+}
+
+
 class Habit(Base):
     __tablename__ = "habit"
 
@@ -34,6 +57,9 @@ class Habit(Base):
         Enum(HabitStatus, native_enum=False, length=20), nullable=False, default=HabitStatus.ACTIVE
     )
     weekdays_mask: Mapped[int] = mapped_column(SmallInteger, nullable=False, default=ALL_WEEKDAYS_MASK)
+    difficulty: Mapped[HabitDifficulty] = mapped_column(
+        Enum(HabitDifficulty, native_enum=False, length=20), nullable=False, default=HabitDifficulty.MEDIUM
+    )
     condition_text: Mapped[str | None] = mapped_column(String(300), nullable=True)
     reminder_time: Mapped[time | None] = mapped_column(Time, nullable=True)
     sort_order: Mapped[int | None] = mapped_column(Integer, nullable=True)
@@ -48,3 +74,15 @@ class Habit(Base):
     def is_scheduled_on(self, weekday: int) -> bool:
         """weekday: Python date.weekday() 기준 (월=0 ... 일=6)"""
         return bool(self.weekdays_mask & (1 << weekday))
+
+    @property
+    def target_days(self) -> int | None:
+        return HABIT_DIFFICULTY_TARGET_DAYS[self.difficulty]
+
+    @property
+    def goal_target_date(self) -> date | None:
+        """목표 기간이 끝나는 날짜. UNLIMITED면 None."""
+        days = self.target_days
+        if days is None:
+            return None
+        return self.created_at.date() + timedelta(days=days - 1)