- Add backend/internal/s3/ with build-tag pattern (dev vs prod)
- Dev: Uses local Rustfs container (S3-compatible)
- Prod: Stub for R2 Cloudflare (add AWS SDK to implement)
- Add S3 env vars to .env.example and .env
- Add Rustfs service to compose.yml
- Add Rustfs reset to local-dev-2.sh (wipes data on each run)