feat: explain activity data quality

apps/api/tests/test_api.py

@@ -46,11 +46,12 @@ def test_activity_filters_and_explains_score() -> None:
     assert len(payload) == 1
     assert payload[0]["catches"] == 3
     assert payload[0]["unique_players"] == 3
-    assert "3 свежих уловов" in payload[0]["explanation"]
+    assert "3 свежих улова" in payload[0]["explanation"]
 
 
 def test_invalid_period_is_rejected() -> None:
     assert client.get("/api/v1/activity?hours=13").status_code == 422
+    assert client.get("/api/v1/activity?sort=unknown").status_code == 422
 
 
 def test_spot_detail_and_catches() -> None: