Add optional PostgreSQL service to `docker-compose.yml`

[peteski22]

Part of #257 — Phase 4 step (3).
RFC: #275.

Blocked by the PostgresStore implementation Phase 2 child.

Scope

Add an optional PostgreSQL service to docker-compose.yml that the
cq server can be pointed at via CQ_DATABASE_URL. SQLite remains the
default — the Postgres service is opt-in (e.g. via a profile or a
documented --profile postgres flag).

Definition of Done

• docker compose up (no profile) starts the server in default
  SQLite mode unchanged.
• docker compose --profile postgres up (or equivalent) starts both
  the Postgres service and the server pointed at it via
  CQ_DATABASE_URL.
• README / docker docs updated with the two invocations.
• Lint clean.