(feature) Add run completion notifications

Add email and webhook notification targets with delivery tracking, and send
notifications when backup runs reach a terminal status.

Expose notification target management in the Django UI and keep delivery
failures recorded without failing the backup worker.

src/pobsync_server/urls.py

@@ -13,6 +13,9 @@ urlpatterns = [
     path("changelog/", views.changelog, name="changelog"),
     path("self-check/", views.self_check, name="self_check"),
     path("logs/", views.logs, name="logs"),
+    path("notifications/", views.notification_targets, name="notification_targets"),
+    path("notifications/new/", views.create_notification_target, name="create_notification_target"),
+    path("notifications/<int:target_id>/", views.edit_notification_target, name="edit_notification_target"),
     path("purged-snapshots/", views.purged_snapshots, name="purged_snapshots"),
     path("schedules/", views.schedules_list, name="schedules_list"),
     path("config/global/", views.edit_global_config, name="edit_global_config"),