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.

tests/test_api_habits.py

@@ -70,6 +70,18 @@ def test_complete_and_reactivate_habit(auth_client, db_session, test_user):
     assert reactivate_res.json()["status"] == "active"
 
 
+def test_abandon_and_reactivate_habit(auth_client, db_session, test_user):
+    habit = _make_habit(db_session, test_user.id)
+
+    abandon_res = auth_client.post(f"/api/habits/{habit.id}/abandon")
+    assert abandon_res.status_code == 200
+    assert abandon_res.json()["status"] == "abandoned"
+
+    reactivate_res = auth_client.post(f"/api/habits/{habit.id}/reactivate")
+    assert reactivate_res.status_code == 200
+    assert reactivate_res.json()["status"] == "active"
+
+
 def test_reorder_endpoint(auth_client, db_session, test_user):
     a = _make_habit(db_session, test_user.id, name="A")
     b = _make_habit(db_session, test_user.id, name="B")