Add costs to each row in yearly usage API

This will replace the manual calculations in Admin [^1][^2] for SMS
and also in API [^3] for annual letter costs.

Doing the calculation here also means we correctly attribute free
allowance to the earliest rows in the billing table - Admin doesn't
know when a given rate was applied so can't do this without making
assumptions about when we change our rates.

Since the calculation now depends on annual billing, we need to
change all the tests to make sure a suitable row exists. I've also
adjusted the test data to match the assumption that there can only
be one SMS rate per bst_date.

Note about "OVER" clause
========================

Using "rows=" ("ROWS BETWEEN") makes more sense than "range=" as
we want the remainder to be incremental within each group in a
"GROUP BY" clause, as well as between groups i.e

  # ROWS BETWEEN (arbitrary numbers to illustrate)
  date=2021-04-03, units=3, cost=3.29
  date=2021-04-03, units=2, cost=4.17
  date=2021-04-04, units=2, cost=5.10

  vs.

  # RANGE BETWEEN
  date=2021-04-03, units=3, cost=4.17
  date=2021-04-03, units=2, cost=4.17
  date=2021-04-04, units=2, cost=5.10

See [^4] for more details and examples.

[^1]: https://github.com/alphagov/notifications-admin/blob/master/app/templates/views/usage.html#L60
[^2]: 072c3b2079/app/billing/billing_schemas.py (L37)
[^3]: 474d7dfda8/app/templates/views/usage.html (L98)
[^4]: https://learnsql.com/blog/difference-between-rows-range-window-functions/

tests/app/billing/test_rest.py

@@ -147,6 +147,8 @@ def set_up_monthly_data():
                               billable_unit=1,
                               rate=0.33,
                               postage='second')
+
+    create_annual_billing(service_id=service.id, free_sms_fragment_limit=4, financial_year_start=2016)
     return service
 
 
@@ -174,6 +176,7 @@ def test_get_yearly_usage_by_monthly_from_ft_billing(admin_request, notify_db_se
     assert letter_row["notifications_sent"] == 30
     assert letter_row["rate"] == 0.33
     assert letter_row["postage"] == "second"
+    assert letter_row["cost"] == 9.9
 
     assert sms_row["month"] == "April"
     assert sms_row["notification_type"] == "sms"
@@ -182,6 +185,8 @@ def test_get_yearly_usage_by_monthly_from_ft_billing(admin_request, notify_db_se
     assert sms_row["notifications_sent"] == 30
     assert sms_row["rate"] == 0.162
     assert sms_row["postage"] == "none"
+    # free allowance is 4, so (30 - 4) * 0.162
+    assert sms_row["cost"] == 4.212
 
 
 def set_up_yearly_data():
@@ -189,6 +194,7 @@ def set_up_yearly_data():
     sms_template = create_template(service=service, template_type="sms")
     email_template = create_template(service=service, template_type="email")
     letter_template = create_template(service=service, template_type="letter")
+
     for month in range(1, 13):
         mon = str(month).zfill(2)
         for day in range(1, monthrange(2016, month)[1] + 1):
@@ -209,6 +215,8 @@ def set_up_yearly_data():
                               rate=0.33,
                               postage='second')
         start_date, end_date = get_month_start_and_end_date_in_utc(datetime(2016, int(mon), 1))
+
+    create_annual_billing(service_id=service.id, free_sms_fragment_limit=4, financial_year_start=2016)
     return service
 
 
@@ -251,6 +259,7 @@ def test_get_yearly_billing_usage_summary_from_ft_billing(admin_request, notify_
     assert json_response[0]['notifications_sent'] == 275
     assert json_response[0]['rate'] == 0
     assert json_response[0]['letter_total'] == 0
+    assert json_response[0]['cost'] == 0
 
     assert json_response[1]['notification_type'] == 'letter'
     assert json_response[1]['billing_units'] == 275
@@ -258,6 +267,7 @@ def test_get_yearly_billing_usage_summary_from_ft_billing(admin_request, notify_
     assert json_response[1]['notifications_sent'] == 275
     assert json_response[1]['rate'] == 0.33
     assert json_response[1]['letter_total'] == 90.75
+    assert json_response[1]['cost'] == 90.75
 
     assert json_response[2]['notification_type'] == 'sms'
     assert json_response[2]['billing_units'] == 825
@@ -265,3 +275,4 @@ def test_get_yearly_billing_usage_summary_from_ft_billing(admin_request, notify_
     assert json_response[2]['notifications_sent'] == 550
     assert json_response[2]['rate'] == 0.0162
     assert json_response[2]['letter_total'] == 0
+    assert json_response[2]['cost'] == 13.3002