M4: Cloudflare R2 Storage Setup (litmus-j0d6)

Create R2 bucket litmus-screenshots with public access and custom domain. Configure CORS. Document URL scheme: {base_url}/v1/{provider}/{theme}/{fixture}.webp. Add R2 credentials to GitHub Actions secrets. Note: manual infra step, document the process.

Summary of Changes

M4 is primarily a manual infrastructure setup task. The codebase is ready for R2 integration — the manifest model, URL scheme, and upload hooks are designed.

Setup steps for the user:

1. Create R2 bucket litmus-screenshots in Cloudflare dashboard
2. Enable public access with custom domain screenshots.litmus.edger.dev
3. Configure CORS:
   • Allowed origin: https://litmus.edger.dev
   • Allowed methods: GET
4. Add GitHub Actions secrets:
   • R2_ACCESS_KEY_ID
   • R2_SECRET_ACCESS_KEY
   • R2_ACCOUNT_ID
   • R2_ENDPOINT (e.g. https://${ACCOUNT_ID}.r2.cloudflarestorage.com)
   • LITMUS_SCREENSHOTS_BASE_URL (e.g. https://screenshots.litmus.edger.dev)

URL scheme implemented: {base_url}/v1/{provider}/{theme}/{fixture}.webp

Cache headers to configure on the bucket:

• Images: Cache-Control: public, max-age=31536000, immutable
• manifest.json: Cache-Control: public, max-age=3600

No code changes needed for M4 — the data model and CLI upload command are ready in M3.