Finish Docker demo: WP-CLI setup script, seed, /demo picker page

[mpge]

Tracking issue for PR #25. Compose mounts the plugin into a stock WordPress image; needed to finish:

• docker/entrypoint.sh running wp-cli: core install, user create (alice/bob/carol/frank/grace), plugin activate, seed demo tickets (ideally via an 'escalated' wp-cli subcommand)
• /demo WordPress page template or custom route that lists the seeded users and uses wp_set_auth_cookie() for click-login
• reset-on-up so the demo is reproducible (wp db reset --yes then re-run setup)

Note: this demo is on MariaDB (not Postgres) because WordPress is MySQL-family.