feat(deploy): templatize systemd services on install_dir via Jinja2

Distros reserve /opt for different things (some package managers own it
outright), and a DECNET install that wants to live at /srv/decnet or
/usr/local/decnet had to hand-edit 13 service files post-install.

Converts every deploy/decnet-*.service to a .j2 template keyed on
{{ install_dir }}, rendered by `decnet init` at install time. All other
paths (log_dir, state_dir, runtime_dir, user, group) stay standard —
only install_dir varies.

Changes:
- deploy/decnet-*.service → deploy/decnet-*.service.j2 (13 files).
- decnet init gains --install-dir (default /opt/decnet, preserves
  existing behaviour byte-for-byte). Validates absolute-path at the
  CLI boundary. Threads through useradd --home-dir and the dir-creation
  list so the filesystem layout matches the rendered templates.
- _install_units renders via Jinja2 with StrictUndefined (typo → loud
  error, not a silent broken unit). SHA over rendered output so
  operators with a custom install_dir get idempotent re-runs.
- decnet.target, tmpfiles.d, polkit rule stay static — they don't
  reference install paths.
- 4 new tests: custom install_dir renders into units, default remains
  /opt/decnet, relative paths rejected, second run with same custom
  dir is idempotent.

deploy/decnet-forwarder.service.j2

@@ -0,0 +1,46 @@
+[Unit]
+Description=DECNET Syslog-over-TLS Forwarder (worker, RFC 5425)
+Documentation=https://git.resacachile.cl/anti/DECNET/wiki/Logging-and-Syslog
+After=network-online.target
+Wants=network-online.target
+# The forwarder can run independently of the agent — it only needs the local
+# log file to exist and the master to be reachable.
+
+[Service]
+Type=simple
+User=decnet
+Group=decnet
+WorkingDirectory={{ install_dir }}
+EnvironmentFile=-{{ install_dir }}/.env.local
+# Replace <master-host> with the master's LAN address or hostname. The agent
+# cert bundle at /etc/decnet/agent is reused — the forwarder presents the same
+# worker identity when it connects to the master's listener.
+ExecStart={{ install_dir }}/venv/bin/decnet forwarder \
+    --log-file /var/log/decnet/decnet.log \
+    --master-host ${DECNET_SWARM_MASTER_HOST} \
+    --master-port 6514 \
+    --agent-dir /etc/decnet/agent
+
+# TLS client connection; no special capabilities.
+CapabilityBoundingSet=
+AmbientCapabilities=
+
+# Security Hardening
+NoNewPrivileges=yes
+ProtectSystem=full
+ProtectHome=read-only
+PrivateTmp=yes
+ProtectKernelTunables=yes
+ProtectKernelModules=yes
+ProtectControlGroups=yes
+RestrictSUIDSGID=yes
+LockPersonality=yes
+# Reads the tailed log; writes a small byte-offset state file alongside it.
+ReadWritePaths=/var/log/decnet
+ReadOnlyPaths=/etc/decnet
+
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=multi-user.target