POST/PUT /apps let any developer set root_path, compose_file,
systemd_unit and managed_by, and store a raw name. Root-run code later
runs docker compose from that root, renders the name into an nginx
vhost and joins it onto sites-available for rm -f. The host-binding
fields are now admin-only (as /manual already was), create slugs the
name and checks for clashes, and a rename must already be a slug.
The nginx site methods all share write_vhost's plain-filename check, so
enable/disable/delete can no longer be pointed outside the site dirs.