Expand the systemd installer so it can perform a complete native
installation with sensible defaults: copy the checkout into the target
app directory, create runtime directories, write the environment file,
install dependencies, configure systemd units, and optionally configure
nginx.
Add a staff-only Django self-check page that verifies runtime settings,
required binaries, writable paths, database connectivity, global config
state, and systemd service status when available.
Document installer overrides and expose the self-check from the main
navigation.
Make native systemd services the recommended production path for
pobsync while keeping Docker Compose available for development and
optional test installs.
Add web, worker, and scheduler systemd unit templates, a native
environment example, an optional nginx reverse proxy template, and an
installer that creates the venv, service user, env file, units, and
runs migrations/static collection.
Allow native deployments to configure POBSYNC_BACKUP_ROOT directly and
document the new production layout and update flow.
