Add "cost" field to monthly usage API

This starts to replace the calculation in Admin [^1] and, similar
to the yearly API, also correctly attributes free allowance when
we have a rate change during a month.

[^1]: 474d7dfda8/app/templates/views/usage.html (L98)

app/billing/billing_schemas.py

@@ -22,6 +22,7 @@ def serialize_ft_billing_remove_emails(rows):
             "chargeable_units": row.chargeable_units,
             "rate": float(row.rate),
             "postage": row.postage,
+            "cost": float(row.cost),
         }
         for row in rows
         if row.notification_type != 'email'

app/dao/fact_billing_dao.py

@@ -244,6 +244,7 @@ def fetch_monthly_billing_for_year(service_id, year):
                 query.c.rate.label("rate"),
                 query.c.postage.label("postage"),
                 query.c.notification_type.label("notification_type"),
+                func.sum(query.c.cost).label("cost"),
             ).group_by(
                 query.c.rate,
                 query.c.notification_type,