- Remove extended columns (repo_id, issue_id, pull_request_id,
channel_id, message_id, target_type, target_id, action_url,
priority, metadata, deleted_at) from CREATE TABLE in 001_init.sql
- Add migration 013_notification_extra_columns.sql that uses
ALTER TABLE ADD COLUMN IF NOT EXISTS for all extended columns
- Move extended column indexes to the new migration
- Ensures compatibility with databases that already have a basic
notification table from a previous schema version
zhenyi