post-setup.sh: follow-up improvements from code review

[riccjohn]

Follow-up items from the review of #9.

Issues

• set -euo pipefail + || echo "" conflict — Pi-hole grep -oP runs in a subshell where a non-zero exit (no match) can abort the script despite the fallback. Fix:

  PIHOLE_API_KEY=$(docker exec pihole grep -oP '(?<=app_password = ")[^"]+' \
    /etc/pihole/pihole.toml 2>/dev/null) || PIHOLE_API_KEY=""

• Pi-hole logout body — DELETE /api/auth passes {"sid": "..."} as JSON body, but Pi-hole v6 doesn't expect a body for logout. Currently masked by || true so harmless, but should be corrected.

• Jellyfin sort_by(.DateCreated) assumption — Relies on ISO 8601 string sort being equivalent to chronological sort. Add a comment noting this assumption.

Enhancements

• --help flag — Print the script's header comment when invoked with --help or -h.
• Summary at the end — Print which keys were written vs skipped so the user doesn't have to scroll back.
• README: mention Portainer timeout is handled — The old docs warned about Portainer's setup wizard timing out. Worth a one-liner noting the script handles initial account creation so you don't have to race the clock.