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/template_utils.py

@@ -1,8 +1,17 @@
-from app.models.habit import ALL_WEEKDAYS_MASK
+from datetime import date
+
+from app.models.habit import ALL_WEEKDAYS_MASK, HABIT_DIFFICULTY_TARGET_DAYS, Habit, HabitDifficulty
 from app.services.log_service import MILESTONE_STREAKS
 
 _DAY_LABELS = ["월", "화", "수", "목", "금", "토", "일"]
 
+_DIFFICULTY_LABELS: dict[HabitDifficulty, str] = {
+    HabitDifficulty.EASY: "하 (21일)",
+    HabitDifficulty.MEDIUM: "중 (66일)",
+    HabitDifficulty.HARD: "상 (254일)",
+    HabitDifficulty.UNLIMITED: "기간 무제한",
+}
+
 
 def is_milestone_streak(streak: int) -> bool:
     return streak in MILESTONE_STREAKS
@@ -15,6 +24,25 @@ def weekday_label(mask: int) -> str:
     return ", ".join(days) if days else "선택된 요일 없음"
 
 
+def difficulty_label(difficulty: HabitDifficulty) -> str:
+    return _DIFFICULTY_LABELS[difficulty]
+
+
+def goal_progress(habit: Habit) -> dict | None:
+    """습관의 목표 기간 진행 상황. 기간 무제한(UNLIMITED)이면 None."""
+    target_days = HABIT_DIFFICULTY_TARGET_DAYS[habit.difficulty]
+    if target_days is None:
+        return None
+    elapsed = (date.today() - habit.created_at.date()).days + 1  # 시작일을 1일차로 계산
+    elapsed = max(elapsed, 1)
+    return {
+        "elapsed": min(elapsed, target_days),
+        "target": target_days,
+        "remaining": max(target_days - elapsed, 0),
+        "reached": elapsed >= target_days,
+    }
+
+
 def heatmap_opacity(checked_count: int, scheduled_count: int) -> float:
     """월별 캘린더 히트맵 셀의 배경 투명도(0~0.9)를 계산한다."""
     if not scheduled_count: