build(playground): publish a playground image served by Caddy

[joshunrau]

Summary

• Give the playground compose service an image: key (ghcr.io/douglasneuroinformatics/open-data-capture-playground). The release matrix already selects every service with both build and image, so release.yaml needs no edit and the playground image now ships alongside api, gateway and web, tagged latest and the bare version.
• Replace the node + http-server runner with caddy:2.7-alpine serving the static dist/, using a Caddyfile identical to apps/web's (gzip/zstd, SPA fallback to index.html). The image carries no Node.
• Move the build stages from node:iron to node:lts-krypton to satisfy the root engines field, matching the other app Dockerfiles.
• Add the playground package to the Delete Package Versions workflow.
• Update the release playbook (it stated no playground image ships) and apps/playground/AGENTS.md.

Side effect: docker compose --profile playground up now pulls the published image instead of building locally, like the other services.

Verification

• docker compose config through the workflow's jq filter now yields four matrix legs including playground.
• Built the image locally and ran it: 193 MB, no node binary; / and unknown paths return index.html; esbuild.wasm serves as application/wasm with gzip; JS and /runtime/v1/* serve as text/javascript.
• Loaded it in a headless browser: the editor boots, esbuild-wasm initializes and the default instrument renders in the preview. A ~35 KB ?files= share link (pre-fix(playground-url): carry the share link payload in the URL fragment #1539 format) also opened correctly.
• Not verified: the multi-arch (linux/amd64,linux/arm64) build and GHCR push, which first run in the Release workflow. No unit or e2e tests added; nothing in either suite exercises a Dockerfile or compose file.

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