add step 8 to installation that restores file ownership to the invoking
user when running with sudo. also adds cleanup for legacy n8n worker
containers during updates.
- add 08_fix_permissions.sh script to detect real user and fix ownership
- add get_real_user() utility with multiple fallback detection methods
- add cleanup_legacy_n8n_workers() to remove old container naming format
- set restrictive permissions (600) on .env file for security
