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-update.sh

@@ -162,7 +162,7 @@ main() {
   # Step 3: Rebuild container
   log "Step 3: Building new container image"
   cd "$compose_dir"
-  if ! DASHCADDY_COMMIT="$commit" docker compose build --quiet 2>&1; then
+  if ! docker compose build --build-arg DASHCADDY_COMMIT="$commit" --quiet 2>&1; then
     log "ERROR: docker compose build failed, rolling back"
     restore_backup "$api_dir" "$from_version"
     local elapsed=$(( $(date +%s) - start_time ))