Mount the named `db-data` volume at PostgreSQL's data directory
(/var/lib/postgresql/data) on the db service and declare it in the
top-level volumes map, so the database survives `docker compose
restart` and `docker compose down` + `up -d` (recreate). Reset with
`docker compose down -v` per the issue rollback note. Header comments
in compose.yaml and the server Dockerfile updated: T04 is no longer out
of scope; T05/T06 remain.
implementer
3dd80f91fe