(bugfix) Mark retention failures as backup warnings

Add a warning status for BackupRun records so successful snapshots are not
reported as failed when post-run SQL retention fails.

Keep the prune error in the run result, link the successful snapshot, and
let the management command complete with a warning instead of raising a
backup failure.

Include warning runs in backup trend summaries and add a regression test
for successful backups with failed retention cleanup.

src/pobsync_backend/tests/test_run_backup_records_snapshot.py

@@ -102,7 +102,7 @@ class RunBackupRecordsSnapshotTests(TestCase):
         self.assertEqual(run.status, BackupRun.Status.SUCCESS)
         self.assertEqual(run.result["prune"], {"ok": True, "source": "sql", "deleted": []})
 
-    def test_prune_failure_is_recorded_on_backup_run(self) -> None:
+    def test_prune_failure_marks_backup_run_as_warning(self) -> None:
         with TemporaryDirectory() as tmp:
             backup_root = Path(tmp) / "backups"
             GlobalConfig.objects.create(name="default", backup_root=str(backup_root))
@@ -128,19 +128,20 @@ class RunBackupRecordsSnapshotTests(TestCase):
                 }
                 retention_apply.side_effect = ConfigError("Deletion blocked by --max-delete=0")
 
-                with self.assertRaises(ConfigError):
-                    call_command(
-                        "run_pobsync_backup",
-                        host.host,
-                        prefix=str(Path(tmp) / "home"),
-                        prune=True,
-                        prune_max_delete=0,
-                        stdout=StringIO(),
-                    )
+                output = StringIO()
+                call_command(
+                    "run_pobsync_backup",
+                    host.host,
+                    prefix=str(Path(tmp) / "home"),
+                    prune=True,
+                    prune_max_delete=0,
+                    stdout=output,
+                )
 
         run = BackupRun.objects.get()
-        self.assertEqual(run.status, BackupRun.Status.FAILED)
+        self.assertEqual(run.status, BackupRun.Status.WARNING)
         self.assertIsNotNone(run.snapshot)
+        self.assertIn("completed with warnings", output.getvalue())
         self.assertEqual(run.result["prune"]["ok"], False)
         self.assertEqual(run.result["prune"]["type"], "ConfigError")
         self.assertEqual(run.result["prune"]["error"], "Deletion blocked by --max-delete=0")