Migrate /cdn/archive.org JS proxy endpoint to FastAPI

[bhardwajparth51]

Closes #11966

Technical

Adds openlibrary/fastapi/cdn.py — a new FastAPI router that mirrors the legacy ia_js_cdn handler in openlibrary/plugins/openlibrary/code.py:

1. Strict allowlist: only donate.js and athena.js are accepted; all other filenames return 404
2. Fetches from https://archive.org/includes/{filename} with a 10s timeout
3. Returns Content-Type: text/javascript and Cache-Control: max-age=86400, matching legacy headers exactly
4. Router registered in openlibrary/asgi_app.py; existing templates and service worker references (footer.html, donation_banner.html, service-worker-matchers.js) are unaffected

One noted difference from legacy: The original handler doesn't call raise_for_status() — so it silently returns HTTP 200 with an HTML error page when archive.org responds 4xx/5xx. I've returned 502 instead, which feels more correct. Happy to revert if you'd prefer exact parity.

Changes

File	Change
openlibrary/fastapi/cdn.py	New — FastAPI router
openlibrary/asgi_app.py	Register cdn_router
openlibrary/tests/fastapi/test_cdn.py	New — unit tests

Requirements Checklist

• Route with strict filename validation (donate.js|athena.js)
• Legacy headers matched (Content-Type, Cache-Control)
• Tests added (mocked, no network calls)
• Router registered in asgi_app.py
• Templates and service worker paths unaffected

Testing

docker compose exec web pytest openlibrary/tests/fastapi/test_cdn.py -v

curl -I http://localhost:8080/cdn/archive.org/donate.js   # expect 200, text/javascript, max-age=86400
curl -I http://localhost:8080/cdn/archive.org/evil.js     # expect 404

Stakeholders

@RayBB

[RayBB]

Looks good just a few docs updates

[RayBB]

Everything looks prefect!

One small notes for my future self. Maybe we want to caching so we don't hit archive.org so often. But I'll have to ask for the logs to see if it matters.