feat: replace email flows with in-app notifications

Add recipient notification detail and filtering, a queued admin composer, and contextual automatic titles. Retire email delivery, obsolete auth routes, newsletter data, and related templates and settings.

apps/notifications/tasks.py

@@ -1,8 +1,9 @@
 import logging
 
 from celery import shared_task
+from django.contrib.auth import get_user_model
 
-from apps.notifications.services import RedisNotificationStore
+from apps.notifications.services import RedisNotificationStore, notify_user
 
 logger = logging.getLogger(__name__)
 
@@ -12,3 +13,19 @@ def cleanup_notification_retention():
     removed = RedisNotificationStore.cleanup_expired()
     logger.info("Cleaned up %s expired notifications", removed)
     return removed
+
+
+@shared_task
+def send_manual_notification(user_ids: list[int], title: str, digest: str, body_markdown: str | None = None):
+    users = get_user_model().objects.filter(pk__in=user_ids, is_active=True, is_deleted=False)
+    sent = 0
+    for user_id in users.values_list("pk", flat=True).iterator():
+        notify_user(user_id, {
+            "type": "manual",
+            "title": title,
+            "message": digest,
+            "body_markdown": body_markdown,
+            "level": "info",
+        })
+        sent += 1
+    return sent