Implement client cost attribution

tests/test_models.py

@@ -20,4 +20,12 @@ def test_model_classes_importable():
     assert BurnRate.__tablename__ == "fin_burn_rates"
     assert RunwayProjection.__tablename__ == "fin_runway_projections"
     assert TokenSpend.__tablename__ == "fin_token_spends"
-    assert ServiceCost.__tablename__ == "fin_service_costs"
+    assert ServiceCost.__tablename__ == "fin_service_costs"
+
+
+def test_service_cost_has_external_attribution_seam():
+    columns = ServiceCost.__table__.columns
+    assert columns["client_id"].nullable is True
+    assert columns["application_id"].nullable is True
+    assert columns["app_instance_id"].nullable is True
+    assert columns["cost_attribution_key"].nullable is True