Adds install.sh and uninstall.sh for one-command deployment of Nexus POS
on Debian 12+, Ubuntu 22.04+, and Raspberry Pi OS. The installer handles
PostgreSQL setup, tenant provisioning, systemd service, and nginx reverse
proxy with interactive business configuration.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
