Fix CI: improve security fix and add link checker configuration

- Fix import order (ast before asyncio)
- Remove NameError from exception handling (ast.literal_eval doesn't raise it)
- Add .lycheeignore to exclude intermittently unavailable star-history API
- Update link-check workflow to exclude star-history API and accept 503 status codes

apps/slack_data/slack_mcp_reader.py

@@ -7,10 +7,10 @@ for indexing in LEANN. It supports various Slack MCP server implementations and
 flexible message processing options.
 """
 
+import ast
 import asyncio
 import json
 import logging
-import ast
 from typing import Any, Optional
 
 logger = logging.getLogger(__name__)
@@ -148,7 +148,7 @@ class SlackMCPReader:
                     if match:
                         try:
                             error_dict = ast.literal_eval(match.group(1))
-                        except (ValueError, SyntaxError, NameError):
+                        except (ValueError, SyntaxError):
                             pass
                     else:
                         # Try alternative format
@@ -156,7 +156,7 @@ class SlackMCPReader:
                         if match:
                             try:
                                 error_dict = ast.literal_eval(match.group(1))
-                            except (ValueError, SyntaxError, NameError):
+                            except (ValueError, SyntaxError):
                                 pass
 
                 if self._is_cache_sync_error(error_dict):