compose.yml backend service now reads the root ./.env (which README instructs users to create) instead of the nonexistent backend/.env. Promote R2_ACCESS_KEY/R2_SECRET_KEY/R2_BUCKET/R2_PUBLIC_URL to active vars (prod S3 reads all four via getEnv) and document the webhook URL/signature-key exact-match requirement with fail-closed (503/403) wording.
- 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)