feat: add host-side auto-updater for zero-touch API container rebuilds

When the in-container self-updater downloads a new version, it writes
trigger.json. The new systemd path unit watches for this file and runs
dashcaddy-update.sh, which backs up current API files, copies the new
ones, rebuilds the container, verifies health, and writes result.json.
Automatic rollback on build or health check failure.

Also fixes undefined `isWindows` variable in self-updater.js and adds
DASHCADDY_HOST_UPDATES_DIR env var to the installer's docker-compose
template for correct container-to-host path translation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

dashcaddy-api/scripts/dashcaddy-updater.service

@@ -0,0 +1,12 @@
+[Unit]
+Description=DashCaddy auto-update handler
+After=docker.service
+Requires=docker.service
+
+[Service]
+Type=oneshot
+ExecStart=/opt/dashcaddy/scripts/dashcaddy-update.sh
+TimeoutStartSec=300
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=dashcaddy-update