add abandoned status to distinguish given-up habits from completed ones

Completed and abandoned habits were both lumped under a single terminal
state, making it impossible to tell "finished successfully" apart from
"gave up" when reviewing habit history.

app/models/habit.py

@@ -18,6 +18,7 @@ class HabitType(str, enum.Enum):
 class HabitStatus(str, enum.Enum):
     ACTIVE = "active"
     COMPLETED = "completed"
+    ABANDONED = "abandoned"
 
 
 class Habit(Base):
@@ -38,6 +39,7 @@ class Habit(Base):
     sort_order: Mapped[int | None] = mapped_column(Integer, nullable=True)
     created_at: Mapped[datetime] = mapped_column(server_default=func.now(), nullable=False)
     completed_at: Mapped[datetime | None] = mapped_column(nullable=True)
+    abandoned_at: Mapped[datetime | None] = mapped_column(nullable=True)
 
     logs: Mapped[list["HabitLog"]] = relationship(
         back_populates="habit", cascade="all, delete-orphan", passive_deletes=True