If cluster files lose postgres ownership (stray recursive chown,
interrupted install), the postmaster stays up but every new backend
dies with FATAL: could not open file "global/pg_filenode.map":
Permission denied — and dropdb/psql cannot work either. Restore
ownership of /var/lib/postgresql and /etc/postgresql and restart
postgres before recreating the database, and start the API when done.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>