fix(init): thread --user / --group through systemd unit templates

Every decnet-*.service.j2 hardcoded User=decnet / Group=decnet. The
init CLI accepted --user / --group and used them for useradd,
chown, /etc/decnet ownership and ReadWritePaths — but the Jinja
context omitted them entirely, so

  sudo decnet init --install-dir $PWD --user anti --group anti

rendered

  User=decnet
  Group=decnet

into every unit, which at best ran the workers as a user that didn't
match the files (fails to read the venv / config), and at worst spun
a parallel system user the operator never asked for.

Swap the hardcoded lines to {{ user }} / {{ group }} across all 13
templates and add both to the Jinja context in _install_units.

deploy/decnet-api.service.j2

@@ -7,8 +7,8 @@ Requires=docker.service
 
 [Service]
 Type=simple
-User=decnet
-Group=decnet
+User={{ user }}
+Group={{ group }}
 # docker.sock is group-readable by 'docker'; the API ingester tails container logs.
 SupplementaryGroups=docker
 WorkingDirectory={{ install_dir }}