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>

.woodpecker/api.yml

@@ -17,7 +17,7 @@ when:
         - ".woodpecker/api.yml"
 
 variables:
-  - &node_image "node:20-alpine"
+  - &node_image "node:24-alpine"
   - &install_deps |
     corepack enable
     pnpm install --frozen-lockfile