Phase 0 (PLANS.md section 2) calls for two Postgres instances:
- postgres-main on host port 5432 — the primary read/write target
- postgres-backup on host port 5433 — used by MultiPoolService in
  Phase 3 for read-only fallback and HA failover

The legacy single-postgres service and its postgres_data volume are
renamed so existing local volumes do not collide with the new layout.
Operators with a pre-existing postgres_data volume from earlier work
can rename it manually with `docker volume rename` before pulling.

Depends on feat/env-fix-port-credentials for the DB_READ_* env block
that the backup service consumes. app-dev / app-staging / app-prod
profiles' depends_on entries are updated to point at postgres-main.
docker compose config validates clean.
Co-authored-by: Cursor <cursoragent@cursor.com>