Add weekly new products email and related features

Introduces a weekly summary email for newly added products, including a new email template and Celery periodic task. Adds 'include_in_week_summary_email' to Product and 'newsletter' to CustomUser. Provides an admin endpoint to manually trigger the weekly email, updates Celery Beat schedule, and adds email templates for verification and password reset.

backend/advertisement/urls.py

@@ -1,7 +1,7 @@
 from django.urls import path, include
 from rest_framework.routers import DefaultRouter
 
-from .views import ContactMePublicView, ContactMeAdminViewSet
+from .views import ContactMePublicView, ContactMeAdminViewSet, trigger_weekly_email
 
 router = DefaultRouter()
 router.register(r"contact-messages", ContactMeAdminViewSet, basename="contactme")
@@ -12,4 +12,5 @@ urlpatterns = [
 
     # Admin endpoints
     path("", include(router.urls)),
+    path("trigger-weekly-email/", trigger_weekly_email, name="trigger-weekly-email"),
 ]