Configure tgadmin service account during .deb installation

[Al-Muhandis]

Motivation

• Avoid hardcoding a local username in the packaged service so the daemon can run without root on other systems.
• Make the runtime user/group configurable at package install time so the package is portable between hosts.
• Ensure the package can create a system account when needed and instruct systemd to run the service under that account.

Description

• Add a packaged systemd unit debian/lib/systemd/system/tgadmin.service that does not embed User/Group so runtime identity can be provided via a drop-in.
• Add /etc/default/tgadmin (added as a conffile) with TGADMIN_SERVICE_USER and TGADMIN_SERVICE_GROUP defaulting to tgadmin.
• Update debian/DEBIAN/postinst to read TGADMIN_SERVICE_USER/GROUP from /etc/default/tgadmin, create the system group and user if missing, and write a systemd drop-in /etc/systemd/system/tgadmin.service.d/10-run-as.conf containing User= and Group= so the service runs as the non-root account.
• Adjust config ownership in postinst to chown "root:${SERVICE_GROUP}" and update debian/DEBIAN/postrm purge handling to remove the generated drop-in; add /etc/default/tgadmin to DEBIAN/conffiles.

Testing

• Ran a shell syntax check with bash -n debian/DEBIAN/postinst debian/DEBIAN/postrm which completed successfully.