(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_run_scheduled_config_source.py

@@ -49,6 +49,24 @@ class RunScheduledConfigSourceTests(SimpleTestCase):
         self.assertEqual(result["host"], "web-01")
         run_rsync.assert_called_once()
 
+    def test_failed_dry_run_includes_log_tail(self) -> None:
+        def fake_run_rsync(command, log_path, timeout_seconds):
+            log_path.parent.mkdir(parents=True, exist_ok=True)
+            log_path.write_text("Permission denied (publickey).\nrsync error\n", encoding="utf-8")
+            return RsyncResult(exit_code=12, command=command)
+
+        with patch("pobsync.commands.run_scheduled.run_rsync", side_effect=fake_run_rsync):
+            result = run_scheduled(
+                prefix=Path("/missing-prefix"),
+                host="web-01",
+                dry_run=True,
+                config_source=FakeConfigSource(),
+            )
+
+        self.assertFalse(result["ok"])
+        self.assertEqual(result["rsync"]["exit_code"], 12)
+        self.assertEqual(result["rsync"]["log_tail"], ["Permission denied (publickey).", "rsync error"])
+
     def test_successful_real_run_applies_prune_when_requested(self) -> None:
         with TemporaryDirectory() as tmp:
             prefix = Path(tmp) / "home"