Adds `docker-compose.external-db.yml` so teams can point the stack at a
managed Postgres (RDS, Cloud SQL, etc.) without forking the base compose
file. Disables the bundled `db` service via an unreachable `profiles`
label and replaces `depends_on` / `DATABASE_URL` on `migrator` and `app`
with `!override`-tagged blocks that read `DATABASE_URL` straight from
`.env`. Default `docker compose up -d` flow is unchanged.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
