feat(notifications): add scheduled email notification system

- NotificationSchedule + NotificationRecipient models with initial migration
- emails.py BUILDERS registry + events_expiring_this_week HTML email builder (IST week bounds)
- send_scheduled_notifications management command (croniter due-check + select_for_update(skip_locked))
- 6 admin API endpoints under /api/v1/notifications/ (types, schedules CRUD, recipients CRUD, send-now)
- date_from/date_to filters on EventListView for dashboard card
- croniter>=2.0.0 added to requirements

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

admin_api/urls.py

@@ -81,6 +81,14 @@ urlpatterns = [
     path('rbac/org-tree/', views.OrgTreeView.as_view(), name='rbac-org-tree'),
     path('rbac/audit-log/', views.AuditLogListView.as_view(), name='rbac-audit-log'),
 
+    # Notifications (admin-side recurring email jobs)
+    path('notifications/types/', views.NotificationTypesView.as_view(), name='notification-types'),
+    path('notifications/schedules/', views.NotificationScheduleListView.as_view(), name='notification-schedule-list'),
+    path('notifications/schedules/<int:pk>/', views.NotificationScheduleDetailView.as_view(), name='notification-schedule-detail'),
+    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'),
+
     # Ad Control
     path('ad-control/', include('ad_control.urls')),
 ]