Fixed Tinybird KPI results (#21914)

ref https://linear.app/ghost/issue/ANAL-120/bounce-rate-data-seems-to-mix-units
closes https://linear.app/ghost/issue/ANAL-119/visit-duration-metric-inaccurate
closes https://linear.app/ghost/issue/ANAL-118/charts-are-empty-with-only-1-data-point

- The original [web analytics starter kit KPI endpoint](https://github.com/tinybirdco/web-analytics-starter-kit/blob/ad1efb766e2bfbcc85d6c037cf23f02d36a2d0a1/tinybird/pipes/kpis.pipe#L122) had this simpler endpoint, but as I've messed around adding features, I've unintentionally overcomplicated it and introduced a tonne of bugs.
- This reverts the KPI endpoint back towards the original structure, and moves all the calculations and where statements up to the data node
- This means that the left join at the end works and pulls in all the dates from the timeseries node correctly, without the need for using `WITH FILL STEP 1` which generated a result for every second when looking at a single days data. 
- Moving the where clause handling up to the `data` node, rather than being on the endpoint still works as expected, which confused me when I first started working with tinybird
- This should resolve several bugs we've experienced with the visit duration, with missing data points and empty charts, and perhaps even the bounce rate (but need to look at that more closely)

ghost/tinybird/pipes/kpis.pipe

@@ -185,36 +185,14 @@ DESCRIPTION >
     General KPIs per date, works for both summary metrics and trends charts.
 
 SQL >
+    %
     select
-        site_uuid,
         date,
-        member_status,
-        device,
-        browser,
-        location,
-        source,
-        pathname,
         uniq(session_id) as visits,
         sum(pageviews) as pageviews,
         sum(case when latest_view_aux = first_view_aux then 1 else 0 end) / visits as bounce_rate,
         avg(latest_view_aux - first_view_aux) as avg_session_sec
     from pageviews
-    group by date, site_uuid, member_status, device, browser, location, source, pathname
-
-NODE endpoint
-DESCRIPTION >
-    Join and generate timeseries with metrics
-
-SQL >
-%
-    select
-        a.date as date,
-        sum(b.visits) as visits,
-        sum(b.pageviews) as pageviews,
-        avg(b.bounce_rate) as bounce_rate,
-        sum(b.avg_session_sec) as avg_session_sec
-    from timeseries a
-    left join data b using date
     where
         site_uuid = {{String(site_uuid, 'mock_site_uuid', description="Tenant ID", required=True)}}
 
@@ -224,5 +202,13 @@ SQL >
         {% if defined(source) %} and source = {{ String(source, description="Source to filter on", required=False) }} {% end %}
         {% if defined(location) %} and location = {{ String(location, description="Location to filter on", required=False) }} {% end %}
         {% if defined(pathname) %} and pathname = {{ String(pathname, description="Pathname to filter on", required=False) }} {% end %}
-  group by date
-  order by date WITH FILL STEP 1
+    group by date
+
+NODE endpoint
+DESCRIPTION >
+    Join and generate timeseries with metrics
+
+SQL >
+    select a.date, b.visits, b.pageviews, b.bounce_rate, b.avg_session_sec
+    from timeseries a
+    left join data b using date

ghost/tinybird/tests/all_kpis.test.result

@@ -1,8 +1,8 @@
 "date","visits","pageviews","bounce_rate","avg_session_sec"
-"2100-01-01",3,5,0.3333333333333333,1741
+"2100-01-01",3,5,0.3333333333333333,580.3333333333334
 "2100-01-02",1,3,0,1027
-"2100-01-03",3,7,0,9999
-"2100-01-04",3,7,0.3333333333333333,1716
-"2100-01-05",2,5,0,616
+"2100-01-03",3,7,0,3333
+"2100-01-04",3,7,0.3333333333333333,572
+"2100-01-05",2,5,0,308
 "2100-01-06",2,2,1,0
 "2100-01-07",2,2,1,0