fix: annotate conference list with has_errors, has_warnings, participants_count (#22)

* fix: annotate conference list with has_errors, has_warnings, participants_count

After expand_fields was removed (PR #19), the conference list no longer
includes participants or issues arrays. This broke the dashboard charts
for error/warning icons and participant counts.

Adds lightweight annotations via Exists() subqueries and Count() so the
list includes:
- has_errors (boolean)
- has_warnings (boolean)
- participants_count (integer)

No N+1 queries — all computed in the same SQL query.

* fix: use Subquery for participants_count to avoid filtered join bug

Count('participants') reuses the filtered JOIN when participantId is in
the query string, returning 1 instead of the real total. Switched to a
Subquery that counts independently of the request's participant filter.

Addresses review feedback from Codex and Alberto.

* fix: exclude soft-deleted participants from participants_count

The Subquery used Participant.objects.filter() which bypasses the
is_active=True check, overcounting if any participants were soft-deleted.

Author: Boanerges1996

app/views/conference_view.py

@@ -1,11 +1,14 @@
 import datetime
 
 from django.core.exceptions import ValidationError
+from django.db.models import Count, Exists, IntegerField, OuterRef, Subquery
 
 from ..errors import (INVALID_PARAMETERS, CONFERENCE_NOT_FOUND,
                       MISSING_PARAMETERS, PMError)
 from ..utils import JSONHttpResponse, serialize, paginate_and_serialize
 from ..models.conference import Conference
+from ..models.issue import Issue
+from ..models.participant import Participant
 from .generic_view import GenericView
 
 class ConferencesView(GenericView):
@@ -40,12 +43,31 @@ def filter(cls, request):
             filters['created_at__gt'] = datetime.datetime.fromisoformat(filters.get('created_at__gt'))
 
         try:
-            objs = Conference.filter(**filters)
+            objs = Conference.filter(**filters).annotate(
+                has_errors=Exists(
+                    Issue.objects.filter(conference=OuterRef('pk'), type='e', is_active=True)
+                ),
+                has_warnings=Exists(
+                    Issue.objects.filter(conference=OuterRef('pk'), type='w', is_active=True)
+                ),
+                participants_count=Subquery(
+                    Participant.objects.filter(
+                        conferences=OuterRef('pk'),
+                        is_active=True,
+                    ).order_by().values('conferences').annotate(
+                        cnt=Count('id', distinct=True)
+                    ).values('cnt')[:1],
+                    output_field=IntegerField(),
+                ),
+            )
         except ValidationError:
             raise PMError(status=400, app_error=INVALID_PARAMETERS)
 
         return JSONHttpResponse(
-            content=paginate_and_serialize(request, objs),
+            content=paginate_and_serialize(
+                request, objs,
+                properties=['has_errors', 'has_warnings', 'participants_count'],
+            ),
         )
 
     @classmethod