(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/config_source.py

@@ -41,6 +41,13 @@ def _attach_credential_options(config: dict[str, Any], credential: SshCredential
     if paths.get("known_hosts") and not _has_ssh_option(options, "UserKnownHostsFile"):
         options.append(f"-oUserKnownHostsFile={paths['known_hosts']}")
     ssh["options"] = options
+    config["ssh_credential"] = {
+        "id": credential.pk,
+        "name": credential.name,
+        "identity_file": paths["identity_file"],
+        "generated": credential.generated,
+        "storage": "filesystem" if credential.key_path else "database",
+    }
 
 
 def _materialize_credential(credential: SshCredential) -> dict[str, str]: