Align supervisor pidfile behavior with Solid Queue

[knifecake]

Summary

• change steady_queue.supervisor_pidfile default from tmp/pids/steady_queue_supervisor.pid to None
• add an atexit cleanup hook in Pidfile.setup() so graceful exits remove pidfiles more reliably
• document the new default in README.md and docs/configuration.rst
• add changelog entry under Unreleased
• add pidfile tests for default configuration, stale pidfile replacement, and atexit hook registration

Why

Solid Queue (Ruby) keeps supervisor_pidfile opt-in (nil by default). Making pidfiles enabled by default in containerized environments can cause startup failures due to stale files and PID reuse across restarts. This aligns behavior with upstream and reduces deploy-time flakiness.

Testing

• Ran full test suite locally with PostgreSQL:
  • DB_URL=postgres://steady_queue:steady_queue@localhost:55432/app STEADY_QUEUE_DB_URL=postgres://steady_queue:steady_queue@localhost:55432/queue .venv/bin/python runtests.py
• Result: Ran 158 tests ... OK