Fix auto-update pipeline bugs discovered in e2e testing

- Fix container-to-host path mapping in trigger.json (stagingDir
  was using container path /app/updates/ instead of host path
  /opt/dashcaddy/updates/)
- Fix download race condition: primary download's async unlink
  could delete mirror download's file — use unlinkSync before retry
- Fix DASHCADDY_COMMIT build arg not passed to docker compose build
  (was set as env var, now uses --build-arg)
- Remove MakeDirectory=yes from systemd path unit (was creating
  trigger.json as directory instead of file)
- Remove unused 'tar' npm module import
- Add mirror fallback for tarball downloads (not just version checks)

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

dashcaddy-api/scripts/dashcaddy-updater.path

@@ -4,7 +4,6 @@ Documentation=https://dashcaddy.net
 
 [Path]
 PathChanged=/opt/dashcaddy/updates/trigger.json
-MakeDirectory=yes
 
 [Install]
 WantedBy=multi-user.target