[Epic] Containers & local stack

[richardmhope]

Epic — M0 Foundations

A one-command local dev stack so contributors can run api + engine + postgres + redis.

Scope

• deploy/compose/docker-compose.yml: api, engine, postgres, redis.
• Base Dockerfiles for the api and engine roles.
• Makefile targets: up, down, migrate, seed, test.
• .env.example wiring the env-key secret-store backend.

Acceptance criteria

• make up brings the stack healthy; make down tears it down.
• Engines scale via --scale engine=N.
• The engine container has no Postgres credentials (invariant check).