(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_backend/tests/test_backup_worker.py

@@ -11,7 +11,7 @@ from django.utils import timezone
 from pobsync.util import write_yaml_atomic
 from pobsync_backend.backup_runner import queue_backup_run, reconcile_running_runs
 from pobsync_backend.management.commands.run_pobsync_worker import Command
-from pobsync_backend.models import BackupRun, GlobalConfig, HostConfig, SnapshotRecord
+from pobsync_backend.models import BackupRun, GlobalConfig, HostConfig, NotificationDelivery, NotificationTarget, SnapshotRecord
 
 
 class BackupWorkerTests(TestCase):
@@ -116,6 +116,42 @@ class BackupWorkerTests(TestCase):
         self.assertEqual(run.rsync_exit_code, 24)
         self.assertEqual(run.result["warning"]["category"], "vanished")
 
+    def test_worker_sends_notification_after_completed_run(self) -> None:
+        with TemporaryDirectory() as tmp:
+            backup_root = Path(tmp) / "backups"
+            GlobalConfig.objects.create(name="default", backup_root=str(backup_root))
+            host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
+            NotificationTarget.objects.create(
+                name="ops",
+                channel=NotificationTarget.Channel.EMAIL,
+                email_to="ops@example.test",
+            )
+            snapshot_dir = backup_root / host.host / "scheduled" / "20260519-021500Z__ABCDEFGH"
+            meta_dir = snapshot_dir / "meta"
+            meta_dir.mkdir(parents=True)
+            write_yaml_atomic(meta_dir / "meta.yaml", {"status": "success", "started_at": "2026-05-19T02:15:00Z"})
+            run = queue_backup_run(host=host)
+
+            with (
+                patch("pobsync_backend.backup_runner.run_scheduled") as run_scheduled,
+                patch("pobsync_backend.notifications.send_mail", return_value=1) as send_mail,
+            ):
+                run_scheduled.return_value = {
+                    "ok": True,
+                    "dry_run": False,
+                    "host": host.host,
+                    "snapshot": str(snapshot_dir),
+                    "base": None,
+                    "rsync": {"exit_code": 0},
+                }
+
+                Command()._run_once(prefix=Path(tmp) / "home")
+
+        run.refresh_from_db()
+        self.assertEqual(run.status, BackupRun.Status.SUCCESS)
+        self.assertEqual(NotificationDelivery.objects.get(run=run).status, NotificationDelivery.Status.SENT)
+        send_mail.assert_called_once()
+
     def test_worker_refreshes_heartbeat_while_run_is_active(self) -> None:
         with TemporaryDirectory() as tmp:
             GlobalConfig.objects.create(name="default", backup_root=str(Path(tmp) / "backups"))