Compose v2 canonical filename (compose.yml) is recognized by both
docker compose and podman compose without preference. The previous
docker-compose.yml worked but signalled a Docker-first stance, while
target deployment is Podman 5.8+ rootless.
- Rename backend/docker-compose.yml -> backend/compose.yml.
- backend/README.md `make db-up` comment uses $(CONTAINER) to mirror
the Makefile auto-detect (lines 14-16: docker || podman).
- backend/README.md audit-writer bootstrap snippet hints at podman
fallback explicitly with `command -v` runtime sniff.
- backend/compose.yml comment for audit-writer mentions both runtimes.
No functional change. Makefile $(COMPOSE) target unchanged: Compose v2
discovers compose.yml first in its search order.
