add habit leveling/XP system, explicit fail marking for missed habits, and scrollable habit tabs

- Habits and the account now accumulate XP and levels from check streaks, with
  streak/level-up celebration banners and push notifications.
- The "yesterday missed" banner on /today now lets a habit be explicitly marked
  failed (not just checked done), backed by a new HabitLog.status column so
  completion-rate/streak calculations never count a failed day as done.
- The habit management tabs scroll horizontally on narrow screens instead of
  wrapping to two lines, with a pure-CSS edge shadow indicating more content.

app/models/habit.py

@@ -10,6 +10,16 @@ from app.database import Base
 ALL_WEEKDAYS_MASK = 0b1111111  # 월~일 전부 (bit0=월 ... bit6=일)
 
 
+def _by_value(enum_cls: type[enum.Enum]) -> list[str]:
+    """SQLAlchemy Enum이 DB에 멤버 이름(예: "MEDIUM")이 아니라 값(예: "medium")을 쓰도록 강제한다.
+
+    values_callable을 안 주면 SQLAlchemy는 기본적으로 .name을 저장하는데, 이 앱의 CHECK 제약조건과
+    마이그레이션 기본값(server_default)은 전부 소문자 .value를 기준으로 작성되어 있어 불일치가 생긴다
+    (실제로 difficulty 컬럼에서 이 불일치로 LookupError가 발생한 사례가 있었음).
+    """
+    return [member.value for member in enum_cls]
+
+
 class HabitType(str, enum.Enum):
     BUILD = "build"  # 만들고 싶은 습관
     QUIT = "quit"  # 멈추고 싶은 습관
@@ -52,13 +62,19 @@ class Habit(Base):
         ForeignKey("user.id", ondelete="CASCADE"), nullable=True
     )
     name: Mapped[str] = mapped_column(String(200), nullable=False)
-    habit_type: Mapped[HabitType] = mapped_column(Enum(HabitType, native_enum=False, length=20), nullable=False)
+    habit_type: Mapped[HabitType] = mapped_column(
+        Enum(HabitType, native_enum=False, length=20, values_callable=_by_value), nullable=False
+    )
     status: Mapped[HabitStatus] = mapped_column(
-        Enum(HabitStatus, native_enum=False, length=20), nullable=False, default=HabitStatus.ACTIVE
+        Enum(HabitStatus, native_enum=False, length=20, values_callable=_by_value),
+        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
+        Enum(HabitDifficulty, native_enum=False, length=20, values_callable=_by_value),
+        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)

app/models/habit_log.py

@@ -1,10 +1,17 @@
+import enum
 from datetime import date, datetime
 
-from sqlalchemy import Date, ForeignKey, Integer, UniqueConstraint
+from sqlalchemy import Date, Enum, ForeignKey, Integer, UniqueConstraint
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.sql import func
 
 from app.database import Base
+from app.models.habit import _by_value
+
+
+class HabitLogStatus(str, enum.Enum):
+    DONE = "done"
+    FAILED = "failed"  # /today의 "어제 놓친 습관" 배너에서 사용자가 명시적으로 실패를 확정한 경우
 
 
 class HabitLog(Base):
@@ -14,6 +21,11 @@ class HabitLog(Base):
     id: Mapped[int] = mapped_column(Integer, primary_key=True)
     habit_id: Mapped[int] = mapped_column(ForeignKey("habit.id", ondelete="CASCADE"), nullable=False)
     log_date: Mapped[date] = mapped_column(Date, nullable=False)
+    status: Mapped[HabitLogStatus] = mapped_column(
+        Enum(HabitLogStatus, native_enum=False, length=20, values_callable=_by_value),
+        nullable=False,
+        default=HabitLogStatus.DONE,
+    )
     checked_at: Mapped[datetime] = mapped_column(server_default=func.now(), nullable=False)
 
     habit: Mapped["Habit"] = relationship(back_populates="logs")