feat(notifications): add test-send endpoint for single-address preview

POST /api/v1/notifications/schedules/<pk>/test-send/ accepts {"email": "..."},
renders the schedule's email, delivers to that address only with [TEST] prefix.
Does not touch last_run_at or last_status.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

admin_api/urls.py

@@ -88,6 +88,7 @@ urlpatterns = [
     path('notifications/schedules/<int:pk>/recipients/', views.NotificationRecipientView.as_view(), name='notification-recipient-create'),
     path('notifications/schedules/<int:pk>/recipients/<int:rid>/', views.NotificationRecipientDetailView.as_view(), name='notification-recipient-detail'),
     path('notifications/schedules/<int:pk>/send-now/', views.NotificationScheduleSendNowView.as_view(), name='notification-schedule-send-now'),
+    path('notifications/schedules/<int:pk>/test-send/', views.NotificationScheduleTestSendView.as_view(), name='notification-schedule-test-send'),
 
     # Ad Control
     path('ad-control/', include('ad_control.urls')),

admin_api/views.py

@@ -2783,3 +2783,51 @@ class NotificationScheduleSendNowView(APIView):
             'recipientCount': recipient_count,
             'schedule':       _serialize_schedule(schedule),
         })
+
+
+class NotificationScheduleTestSendView(APIView):
+    """Send a preview of this schedule's email to a single address.
+
+    Does NOT update last_run_at / last_status — purely for previewing content.
+    """
+    permission_classes = [IsAuthenticated]
+
+    def post(self, request, pk):
+        from notifications.models import NotificationSchedule
+        from notifications.emails import BUILDERS
+        from django.conf import settings
+        from django.core.mail import EmailMessage
+        from django.shortcuts import get_object_or_404
+
+        schedule = get_object_or_404(NotificationSchedule, pk=pk)
+
+        email = (request.data.get('email') or '').strip().lower()
+        if not email:
+            return Response({'error': 'email is required'}, status=400)
+
+        builder = BUILDERS.get(schedule.notification_type)
+        if builder is None:
+            return Response(
+                {'error': f'No builder for type: {schedule.notification_type}'},
+                status=400,
+            )
+
+        try:
+            subject, html = builder(schedule)
+            subject = f'[TEST] {subject}'
+            msg = EmailMessage(
+                subject=subject,
+                body=html,
+                from_email=settings.DEFAULT_FROM_EMAIL,
+                to=[email],
+            )
+            msg.content_subtype = 'html'
+            msg.send(fail_silently=False)
+        except Exception as exc:  # noqa: BLE001
+            log('error', f'Test-send failed for schedule #{pk}: {exc}',
+                request=request, user=request.user)
+            return Response({'error': str(exc)}, status=500)
+
+        log('info', f'Test email sent for schedule #{pk} → {email}',
+            request=request, user=request.user)
+        return Response({'ok': True, 'sentTo': email})