fix(#367): migrate Node.js 20 → 24 LTS

Node.js 24 (Krypton) entered Active LTS on 2026-02-09. Update all
Dockerfiles, CI pipelines, and engine constraint from node:20-alpine
to node:24-alpine. Corrected .trivyignore: tar CVEs come from Next.js
16.1.6 bundled tar@7.5.2 (not npm). Orchestrator and API images are
clean; web image needs Next.js upstream fix.

Fixes #367

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

package.json

@@ -5,7 +5,7 @@
   "type": "module",
   "packageManager": "pnpm@10.19.0",
   "engines": {
-    "node": ">=20.0.0"
+    "node": ">=24.0.0"
   },
   "scripts": {
     "build": "turbo run build",