fix(backup): address second round of PR review findings

- api_v3: guard opts_dict with isinstance check after json.loads so a
  non-object JSON payload (null, array, etc.) returns a 400 instead of a
  500 AttributeError
- backup_manager: wrap tmp ZIP creation and os.replace in try/except so
  the .zip.tmp temp file is always removed on any failure
- backup_manager: replace hardcoded Path("/tmp/_zip_check") sentinel in
  validate_backup with a proper tempfile.TemporaryDirectory() so path
  traversal checks are portable and leave no artifacts
- backup_restore.html: detect partial-success responses (plugins_failed or
  errors non-empty) even when status is 'success' and render yellow/warning
  styling and notify instead of green

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web_interface/blueprints/api_v3.py

@@ -1296,6 +1296,8 @@ def backup_restore():
             opts_dict = json.loads(raw_opts)
         except json.JSONDecodeError:
             return jsonify({'status': 'error', 'message': 'Invalid options JSON'}), 400
+        if not isinstance(opts_dict, dict):
+            return jsonify({'status': 'error', 'message': 'options must be an object'}), 400
 
         opts = backup_manager.RestoreOptions(
             restore_config=_coerce_to_bool(opts_dict.get('restore_config', True)),