Set num_returned on $match response for analytics item_count (#856)

* OpenConceptLab/ocl_online#73 | Set num_returned on \$match response

Surfaces the total number of matched items on the
MetadataToConceptsListView (\$match) response so the analytics
middleware can record it as APITransaction.item_count. Before this
fix, \$match transactions always recorded item_count=0 because the
POST handler returned Response(results) with no headers, and
analytics reads item_count from response.headers.num_returned.

num_returned is set to the sum of len(r['results']) across all input
rows — the total matches produced by the request, which is the
denominator the v2 data extraction plan (item 11) wants for
mapper_items_matched_30d / "rows matched per request" metrics.

Scope is \$match only. \$match-scispacy-loinc lives in ocl-scispacy-loinc
and needs a separate fix if we want item counts there too. Rerank view
has different semantics and is out of scope.

Refs #1 (data-extraction-v2 Bundle 2 loose ends).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Trim $match item_count comment

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: paynejd

core/concepts/views.py

@@ -987,7 +987,15 @@ def post(self, request, **kwargs):  # pylint: disable=unused-argument
                 {'detail': 'You are currently in waitlist for $match operation.'},
                 status=status.HTTP_403_FORBIDDEN
             )
-        return Response(self.filter_queryset())
+        results = self.filter_queryset()
+        response = Response(results)
+        # num_returned is picked up by the analytics middleware as
+        # APITransaction.item_count (see ocl_online#73).
+        if isinstance(results, list):
+            response['num_returned'] = sum(
+                len(r.get('results', [])) for r in results if isinstance(r, dict)
+            )
+        return response
 
 
 class RerankConceptsListView(BaseAPIView):