(bugfix) Surface rsync SSH failure details in run results

Include the selected SSH credential metadata and rsync log tail in
dry-run and failed backup results so Django shows the actual SSH or
rsync failure instead of only the exit code.

Warn in host checks when a host still uses database-stored private key
material, making it easier to spot old credentials after switching to
generated filesystem keys.

src/pobsync_backend/tests/test_django_config_source.py

@@ -90,6 +90,7 @@ class DjangoConfigSourceTests(TestCase):
         self.assertIn("-oBatchMode=yes", cfg["ssh"]["options"])
         self.assertIn(f"-oIdentityFile={identity_file}", cfg["ssh"]["options"])
         self.assertIn(f"-oUserKnownHostsFile={known_hosts}", cfg["ssh"]["options"])
+        self.assertEqual(cfg["ssh_credential"]["storage"], "database")
 
     def test_host_ssh_credential_overrides_global_credential(self) -> None:
         global_credential = SshCredential.objects.create(name="global-key", private_key="GLOBAL")
@@ -133,3 +134,4 @@ class DjangoConfigSourceTests(TestCase):
             cfg = DjangoConfigSource().effective_config_for_host("web-01")
 
         self.assertIn(f"-oIdentityFile={identity_file}", cfg["ssh"]["options"])
+        self.assertEqual(cfg["ssh_credential"]["storage"], "filesystem")