[charts] Add global charts aggregated across opted-in users

PROJECT.org

@@ -622,7 +622,7 @@ When a new books arrives inside of a KoReader import, the metadata is not enrich
   source (previously it would crash).
 - Routed =enrich_book_metadata= to the =background= worker queue.
 
-** TODO [#B] There should a global chart aggregator that builds charts across all users :charts:feature:
+** DONE [#B] There should a global chart aggregator that builds charts across all users :charts:feature:
 :PROPERTIES:
 :ID:       ccc257d5-7d35-42b0-9af2-2941b67880fd
 :END:
@@ -636,6 +636,37 @@ bronze). There should also be a toggle in profile settings that allows a user to
 include their scrobbles in global aggregation. If that check box is not selected
 (and it should off by default) that user is not included in the global charts.
 
+*** Implementation
+
+- Global chart records are stored as =ChartRecord= rows with a NULL =user=
+    (the column is already nullable), so they reuse the existing model,
+    upserting, indexing and chart rendering infrastructure.
+- Added =UserProfile.include_in_global_charts= (default off) plus a settings
+    form checkbox; users who do not opt in are excluded from aggregation.
+    Toggling the setting triggers =scrobbles.tasks.rebuild_global_charts_for_user()=
+    which rebuilds every global chart period the user has scrobbles in, so the
+    change is reflected immediately instead of waiting for the next scheduled
+    chart build.
+- Refactored ~vrobbler/apps/charts/utils.py~: extracted the media type config
+    into a module-level =MEDIA_CONFIG= and pulled the aggregation and upsert
+    logic into shared =_aggregate_media()= and =_upsert_chart_records()=
+    helpers used by both =build_charts()= and the new =build_global_charts()=.
+- Added =build_global_charts()= plus =build_global_daily/weekly/monthly/yearly_charts()=
+    wrappers. Global charts aggregate scrobbles from opted-in users only and
+    keep just the top three items (gold, silver, bronze) per media type/period.
+- Wired global chart building into the scheduled Celery tasks
+    (~vrobbler/apps/scrobbles/tasks.py~): =create_yesterdays_charts()=,
+    =rebuild_weekly_charts()=, =rebuild_monthly_charts()= and
+    =rebuild_yearly_charts()= now also build the matching global charts.
+- Added =GlobalChartView= at =/charts/global/= (~charts.GlobalChartView~) with a
+    new ~charts/global_chart_index.html~ template showing the medal top three
+    per media type, with the same year/month/week navigation as user charts.
+- Anonymous users now see the global top-three charts on media detail pages:
+    ~scrobbles.views.ChartContextMixin~ queries =user__isnull=True= records
+    when the request user is anonymous.
+- Tests in ~vrobbler/apps/charts/tests/test_views.py~ and new global chart
+    tests in ~vrobbler/apps/charts/tests/test_utils.py~.
+
 
 ** DONE [#B] Make it possible for anonymous users to view media objects :media:templates:
 :PROPERTIES: