chore(backend): rename docker-compose.yml -> compose.yml + podman notes

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.

backend/README.md

@@ -34,7 +34,7 @@ backend/
 
 ```bash
 make install      # uv venv + pip install -e .[dev]
-make db-up        # docker compose up -d postgres
+make db-up        # $(CONTAINER) compose up -d postgres  (auto-detect docker|podman)
 make db-bootstrap # one-time: create the mimic_audit_writer role (see below)
 make db-migrate   # alembic upgrade head
 make run          # flask run (debug)
@@ -49,7 +49,9 @@ make lint         # ruff + mypy strict
 (decision D-010). For local development, create it manually after `make db-up`:
 
 ```bash
-docker exec -it mimic-postgres psql -U mimic_app -d mimic \
+# Substitute "podman" for "docker" if your runtime is Podman.
+$(command -v docker || command -v podman) exec -it mimic-postgres \
+  psql -U mimic_app -d mimic \
   -c "CREATE ROLE mimic_audit_writer LOGIN PASSWORD 'pick-a-dev-secret';"
 ```