Introduce notifications app and integrate emails

Add a new notifications app (models, serializers, views, admin, tasks, consumers, routing, urls, tests) with Notification.notify for in-app websocket pushes and optional email delivery. Centralize email sending in notifications.tasks.send_email_with_context and re-export it from account.tasks for compatibility. Update account and commerce and advertisement tasks to use Notification.notify/_notify_order and the new email helper; adjust order/notification tasks to consolidate logic. Wire notifications into ASGI routing, settings and URL conf. Misc: handle OSError when importing weasyprint, add tmp/ to .gitignore, add local .claude PowerShell checks, add social.blog skeleton, and remove legacy ews-component test files.

backend/notifications/admin.py

@@ -0,0 +1,11 @@
+from django.contrib import admin
+from .models import Notification
+
+
+@admin.register(Notification)
+class NotificationAdmin(admin.ModelAdmin):
+    list_display = ("id", "title", "notification_type", "user", "is_read", "bulk", "send_email", "created_at")
+    list_filter = ("notification_type", "is_read", "bulk", "send_email", "created_at")
+    search_fields = ("title", "text", "user__email", "user__username")
+    ordering = ("-created_at",)
+    readonly_fields = ("created_at", "read_at", "email_subject", "email_template_path", "send_email", "bulk")