fix: bulk challenge enable path (compose YAML, base images, async toggle, tooling)

- fix_dup_volumes.py: 4 canonical templates had TWO volumes: keys inside one
  service (invalid YAML -> 'mapping key volumes already defined'), which broke
  every enable for anti-alchemy/burvesigner/gemas-notes/kode-viewer.
- fjb: ghcr.io base is not anonymously pullable on this host; swapped to the
  official httpd:2.4 (its httpd.conf only uses stock modules). Added
  onlyBuiltDependencies to package.json (pnpm >=10 blocks esbuild's postinstall).
- xl + kode-viewer: node:20-slim-bookworm is not a real tag; use
  node:20-bookworm-slim. gift-voucher: buster -> bookworm.
- prebuild_images.py: build each challenge's shared services-<name> image once
  in parallel (passes a placeholder PASSWORD build-arg, since several Dockerfiles
  run chpasswd and fail on an empty arg).
- set_enabled.py / sync_all_challenges.py: batch registry flip + runtime apply
  that survives panel restarts and reports per-team results.
- Challenge toggle is now async: PATCH returns a job id, the client polls
  /api/challenges/jobs/<id> so a multi-minute build no longer blocks the panel.
  Added _SYNC_LOCK to serialize concurrent compose rewrites.

services/gift-voucher/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.11-slim-buster
+FROM python:3.11-slim-bookworm
 
 ARG PASSWORD